Use newly-built version of Mill scripts for dist rather than Mill's current ./mill bootstrap script (#5801)

Previously we used the `./mill` in the Mill repo that can potentially be
stale and out of date, e.g. the current one was last updated in
`1.0.0-M1-21-7b6fae-DIRTY892b63e8`, with an old `DEFAULT_MILL_VERSION`.
This PR makes us always use the most recently built script so the
`DEFAULT_MILL_VERSION` is up to date

With the `DEFAULT_MILL_VERSION` fixed, we now have the freedom to not
need a `//| mill-version:` by default, since the bootstrap script comes
with one. The user can still customize the `//| mill-version` or
`.mill-version` later if they want, but this should remove one manual
step from getting started.

We can also recommend people update Mill by always downloading new
bootstrap scripts via `curl`, which removes yet another degree of
freedom and avoids the failure mode we have hit in the past where people
bump `//| mill-version`, but don't download the new bootstrap script
necessary for using the newer version, resulting in the Mill upgrade
failing

Author: lihaoyi

dist/package.mill

@@ -190,36 +190,12 @@ object `package` extends MillJavaModule with DistModule {
     }
   }
 
-  def millBootstrap = Task.Source(BuildCtx.workspaceRoot / "mill")
-  def millBootstrapBat = Task.Source(BuildCtx.workspaceRoot / "mill.bat")
-
-  def prepareBootstrapLauncher(
-      bootstrap: os.Path,
-      dest: os.Path,
-      buildVersion: String,
-      fileName: String
-  ) = {
-    val outputPath = dest / fileName
-    val millBootstrapGrepPrefix = "(\n *DEFAULT_MILL_VERSION=)"
-
-    os.write(
-      outputPath,
-      os.read(bootstrap)
-        .replaceAll(
-          millBootstrapGrepPrefix + "[^\\n]+",
-          "$1" + buildVersion
-        )
-    )
-
-    if (!scala.util.Properties.isWin) os.perms.set(outputPath, "rwxrwxrwx")
-
-    PathRef(outputPath)
-  }
   def bootstrapLauncher = Task {
-    prepareBootstrapLauncher(millBootstrap().path, Task.dest, build.millVersion(), "mill")
+    build.dist.scripts.millSh.compile0()
   }
+
   def bootstrapLauncherBat = Task {
-    prepareBootstrapLauncher(millBootstrapBat().path, Task.dest, build.millVersion(), "mill.bat")
+    build.dist.scripts.millBat.compile0()
   }
 
   def examplePaths: Task[Seq[os.Path]] = Task.Input {
@@ -245,10 +221,6 @@ object `package` extends MillJavaModule with DistModule {
         val ignoreErrorsOnCI = Task.dest / exampleStr / "ignoreErrorsOnCI"
         if (os.exists(ignoreErrorsOnCI)) os.remove(ignoreErrorsOnCI)
         val buildMill = Task.dest / exampleStr / "build.mill"
-        os.write.over(
-          buildMill,
-          s"//| mill-version: ${build.millVersion()}\n" + os.read(buildMill)
-        )
         os.copy(bootstrapLauncher().path, Task.dest / exampleStr / "mill")
         os.copy(bootstrapLauncherBat().path, Task.dest / exampleStr / "mill.bat")
         val zip = Task.dest / s"$exampleStr.zip"

website/docs/modules/ROOT/pages/cli/build-header.adoc

@@ -28,8 +28,10 @@ package build
 
 Mill's build headers can contain the following keys:
 
-1. xref:cli/installation-ide.adoc[mill-version]: this must be unquoted in the first line
-   of the build header, and specifies the version of Mill used in this project.
+1. xref:cli/installation-ide.adoc[mill-version]: if provided, this must be unquoted in
+   the first line of the build header, and specifies the version of Mill used in this
+   project. If not provided, Mill will launch using the version of your `./mill` or
+   `./mill.bat` bootstrap script.
 
 2. xref:#_mill_opts[]: flags to pass to Mill by default, useful for flags that you want
    everyone to use so they don't need to always remember to type them at the command line

website/docs/modules/ROOT/pages/cli/installation-ide.adoc

@@ -22,7 +22,6 @@ bootstrap script, included. You can also download the bootstrap script manually:
 ----
 > curl -L {mill-download-url}/mill-dist-{mill-version}-mill.sh -o mill
 > chmod +x mill
-> echo "//| mill-version: {mill-version}" > build.mill
 > ./mill version
 {mill-version}
 ----
@@ -31,7 +30,6 @@ bootstrap script, included. You can also download the bootstrap script manually:
 [source,console,subs="verbatim,attributes"]
 ----
 > curl.exe -L {mill-download-url}/mill-dist-{mill-version}-mill.bat -o mill.bat
-> echo "//| mill-version: {mill-version}" > build.mill
 > ./mill version
 {mill-version}
 ----
@@ -63,6 +61,10 @@ In-project bootstrap scripts are also useful for running Mill in CI/CD, ensuring
 that your build server like Jenkins or Github Actions has the correct version of Mill
 present to build, compile or test your code.
 
+You can also configure a `//| mill-version` in your xref:cli/build-header.adoc[build header]
+if you want to try a different version, but the best practice when upgrading is to just
+update your bootstrap scripts which ensures the script version and Mill version are compatible.
+
 If you are starting a new project using a Mill bootstrap script, you can use the
 xref:cli/builtin-commands.adoc#_init[mill init] to initialize the project
 folder with one of the Mill example projects. There are a wide range of example projects,