fix(forge): keep the Grails Data plugin off the generated buildscript classpath

[borinquenkid]

What

Grails Forge no longer adds grails-data-hibernate5 / grails-data-hibernate7 to the generated build.gradle buildscript classpath (nor to buildSrc). The plugin stays an ordinary application dependency. GrailsDataHibernate5/7 features and their specs are updated; whatsNew and the 8.0.x upgrade guide (§86) gain a note telling existing builds to drop the entry.

Why

The entry was a leftover from when the CLI found a plugin's commands through the build classpath; commands now ship in companion -cli artifacts that the Grails Gradle plugin discovers from the application's dependencies. With the plugin on the buildscript classpath, grails-datamapping-core's global AST transformations run inside the Groovy that Gradle embeds while compiling build.gradle; once those transformations are compiled by Groovy 6 that fails at configuration time (NoSuchMethodError ... NumberMath.remainder, seen in the Build Grails Forge job: CreateDomainClassCommandSpec and the other command specs).

This is independent of the Java-to-Groovy migration (#16351) but surfaced while splitting it. The buildscript entry has been generated since 8.0.x and the Grails Gradle plugin on every 8.x branch already discovers commands from the application's dependencies, so the fix targets 8.0.x and merges up.

Verification

• Cherry-pick of the fix verified on the migration branch; the Forge build is a separate Gradle build and its specs are run by the Forge CI job (no local test run).

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.9855%. Comparing base (c948a63) to head (a9b6541).

Additional details and impacted files

@@                Coverage Diff                 @@
##                9.0.x     #16575        +/-   ##
==================================================
- Coverage     60.9953%   60.9855%   -0.0099%     
+ Complexity      26642      26640         -2     
==================================================
  Files            2249       2249                
  Lines          111561     111561                
  Branches        20207      20207                
==================================================
- Hits            68047      68036        -11     
- Misses          34527      34537        +10     
- Partials         8987       8988         +1     

see 3 files with indirect coverage changes

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