compile grails-gradle-plugin with the Groovy version provided by Gradle

[jamesfredley]

to ensure build compatibility with Gradle, currently Groovy 3.0.x see: https://docs.gradle.org/current/userguide/compatibility.html#groovy

This will prevent running into Groovy 4 vs 3 compatibility issues like: bertramdev/asset-pipeline#350, when Gradle executes the build using it's embedded Groovy Version, currently 3.0.22.

This will also be done on grails-gradle-model, grails-shell and grails-bootstrap in grails-core. grails/grails-core#13653

Move all versions numbers to gradle.properties and update dependencies to latest compatible version

Simplify ext."signing.*"

[matrei]

Is this a safe-guard for future changes to the build?
Because, as far as I can tell Groovy 4 is not on the compileClasspath right now.

./gradlew dI --dependency groovy --configuration compileClasspath

> Task :dependencyInsight
No dependencies matching given input were found in configuration ':compileClasspath'

This would also need to be done on grails-bootstrap, grails-gradle-model and grails-shell.

These however, do have Groovy 4 on the compileClasspath, and grails-shell in particular also has org.apache.groovy:groovy-ant, org.apache.groovy:groovy-json, org.apache.groovy:groovy-jmx and org.apache.groovy:groovy-xml in api scope which, I believe, is why we need this in the grails-gradle-plugin build.gradle:

// resolve conflict from Gradle's version of Groovy. Not needed if Gradle switches to 4.0
configurations.configureEach { exclude group: 'org.apache.groovy' }

[jamesfredley]

Yes, this change plus similar compile changes on grails-bootstrap, grails-gradle-model and grails-shell will be a safeguard against future changes that could result in a release that breaks build and is not caught until after release. This change can/will be limited to compileOnly for these 4 projects.

Recent changes to grails-gradle-plugin had us move to groovy 4 and then back to 3. The need to ensure Groovy 3 is used to compile was illustrated by the asset-pipeline-gradle:5.0.0 release and it seemed like a good idea to do the same in Grails.