feat!: remove the backup export/import

[gharlan]

Removes the backup export/import from the core without replacement.

Why

• Backups are runtime data, but the core isn't a backup tool: a PHP dumper running in a web request (timeouts, upload/memory limits, no consistency between DB and files, half-imported databases on failure) suggests a safety it can't deliver. mariadb-dump/mysqldump, hosting backups, phpMyAdmin/Adminer and tar/rsync do this better.
• The file import extracted an uploaded tar into the project directory — a backend feature writing arbitrary files into the code base, the very thing 6.x moved away from by taking templates/modules out of the backend.
• The DB import insisted on a dump of the same major version, while with console migrate a dump from any earlier state is no problem at all.
• It can be recreated as addon, but should not be part of the core

Replacement

Import a dump with the database client, then run console migrate:

mysql <db> < dump.sql
php bin/console migrate

Removed

• Backend page Backup (export, upload/server import) and the backup[export] permission
• Redaxo\Core\Backup\Backup, Tar, FileCompressor and the extension points BACKUP_BEFORE_DB_IMPORT/BACKUP_AFTER_DB_IMPORT
• The cronjob type ExportType; existing jobs of that type show up as invalid in the cronjob list
• console setup --db-setup=import and --db-import
• splitbrain/php-archive
• The now unused SQL dump helpers Database\Util::importDump()/splitSqlFile() and Sql::showCreateTable() (separate commit)

Other

• Visual tests: the second setup run (only there to import the fixture) is gone. The instance installed via the migrate endpoint gets the fixture through the mysql client; core.lang moved into the fixture, as it was the only thing the second setup added.
• merge-5x removes redaxo/src/addons/backup/ wholesale on merges from 5.x, like the former install addon.
• Leftover backup files in var/data/core/backup/ of existing instances are not cleaned up.