V4.1.0: CI improvements, bugfixes and cleanup

What's Changed

Added

• Add tests for the model property validations (#2127)
• Split PHPStan CI jobs (#2050)
• Add a more comprehensive XLIFF integrity checker (#2148)
• Add a German translation for the BE module (#2163)
• Add Postgres/MySQL to the functional tests on CI (#2101)
• Add Composer caching on CI (#1933, #2139)

Changed

• Disable some Type Perfect PHPStan rules (#2134)
• Do not process Build/ with Rector (#2126)
• Move PHPCov from Phive to Composer (#2109)

Removed

• Drop Composer scripts for functional tests
• Drop the AbstractFrontendControllerTestCase (#2076)

Fixed

• Fix locallang files (#2158)
• Fix path to the Composer cache in GitHub Actions (#2139)
• Actually fix the code with runTests.sh -s phpCsFixer (#2136)
• Return exitcode != 0 when runTests.sh -s composerUpdate(Min|Max)
  fails (#1936)

New Contributors

• @drc-ravisachaniya made their first contribution in #2073
• @hermannatz made their first contribution in #2127

Full Changelog: v4.0.0...v4.1.0