Revert gradle-groovy.version to hard-coded 4.0.31

Reverts cc2f85f. `GroovySystem.version` returns Gradle's bundled
Groovy, which can lag the version grails-gradle code resolves
transitively via grails-core. The `validateDependencyVersions` task
in CI failed because the BOM advertised Gradle's bundled version while
the modules actually resolved grails-core's:

  org.apache.groovy:groovy           - resolved <core>, expected <gradle-bundled>
  org.apache.groovy:groovy-ant       - resolved <core>, expected <gradle-bundled>
  org.apache.groovy:groovy-groovydoc - resolved <core>, expected <gradle-bundled>
  org.apache.groovy:groovy-templates - resolved <core>, expected <gradle-bundled>
  ... etc

Going back to the hard-coded literal so the BOM matches what we
actually publish. Added an inline comment explaining why
`GroovySystem.version` is not viable here (without specific version
numbers, since those drift every Gradle/Groovy bump).

Verified: `./gradlew :grails-gradle-plugins:validateDependencyVersions
:grails-gradle-tasks:validateDependencyVersions` and
`./gradlew validateDependencyVersions` (whole project, 157 tasks)
both pass.

See #15365 (comment)
for the discussion thread.

Assisted-by: claude-code:claude-opus-4-7

Author: jamesfredley

dependencies.gradle

@@ -29,7 +29,12 @@ ext {
             'byte-buddy.version'            : '1.18.8',
             'commons-text.version'          : '1.15.0',
             'directory-watcher.version'     : '0.19.1',
-            'gradle-groovy.version'         : GroovySystem.version,
+            // Hard-coded rather than GroovySystem.version: the latter returns Gradle's
+            // bundled Groovy, which can lag the version grails-gradle code resolves
+            // transitively via grails-core. validateDependencyVersions requires the
+            // BOM-declared version to match what we actually resolve, so this needs to
+            // track grails-core's chosen Groovy, not Gradle's.
+            'gradle-groovy.version'         : '4.0.31',
             'gradle-spock.version'          : '2.3-groovy-4.0',
             'grails-publish-plugin.version' : '1.0.0-SNAPSHOT',
             'jansi.version'                 : '2.4.2',