Make spotless a convention plugin, and fix google java format issue #3473
Description
After #3446, Spotless is still applied in the root subproject section. This should be converted to a convention plugin.
Also, we need to fix the issue with googlejavaformat on JDK 21.
> Task :mockito-core:spotlessJava FAILED
Step 'google-java-format' found problem in 'src/main/java/org/mockito/invocation/DescribedInvocation.java':
class com.google.googlejavaformat.java.JavaInput (in unnamed module @0xfe86b99) cannot access class com.sun.tools.javac.parser.Tokens$TokenKind (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.parser to unnamed module @0xfe86b99
java.lang.IllegalAccessError: class com.google.googlejavaformat.java.JavaInput (in unnamed module @0xfe86b99) cannot access class com.sun.tools.javac.parser.Tokens$TokenKind (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.parser to unnamed module @0xfe86b99
at com.google.googlejavaformat.java.JavaInput.buildToks(JavaInput.java:350)
at com.google.googlejavaformat.java.JavaInput.buildToks(JavaInput.java:335)
at com.google.googlejavaformat.java.JavaInput.<init>(JavaInput.java:277)
at com.google.googlejavaformat.java.Formatter.getFormatReplacements(Formatter.java:260)
at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:247)
at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:213)
at com.google.googlejavaformat.java.Formatter$formatSource$0.call(Unknown Source)
at spotless_c497s6tbmd544d1gnegbe7k20$_run_closure1$_closure2$_closure3.doCall$original(/Users/brice.dutheil/opensource/mockito/gradle/spotless.gradle:27)
at spotless_c497s6tbmd544d1gnegbe7k20$_run_closure1$_closure2$_closure3.doCall(/Users/brice.dutheil/opensource/mockito/gradle/spotless.gradle)