Default to minimum required Java version for Groovy 5 and 6

Groovy 5 requires Java 11+ and Groovy 6 requires Java 17+. Instead of
hardcoding -DjavaVersion in the allVariants scripts (which would override
an explicit -DjavaVersion=21 passed by the user), build.gradle now silently
defaults to the minimum required Java version when none was specified.
An explicit but incompatible javaVersion still fails loudly.

allVariants and allVariants.bat use a simple loop over all variants,
so e.g. `allVariants -DjavaVersion=21` works correctly for all.

Author: leonard84

allVariants

@@ -1,4 +1,4 @@
 #!/bin/sh
-for var in 2.5 3.0 4.0 5.0; do
+for var in 2.5 3.0 4.0 5.0 6.0; do
     ./gradlew -Dvariant="$var" "$@"
 done

allVariants.bat

@@ -1,4 +1,4 @@
 @echo off
-for %%v in (2.5 3.0 4.0 5.0) do (
+for %%v in (2.5 3.0 4.0 5.0 6.0) do (
     gradlew.bat -Dvariant=%%v %*
 )

build.gradle

@@ -59,7 +59,10 @@ ext {
     minGroovyVersion = "6.0.0"
     maxGroovyVersion = "6.9.99"
     if (javaVersion < 17) {
-      throw new InvalidUserDataException("Groovy $variant is not compatible with Java $javaVersion")
+      if (System.getProperty("javaVersion") != null) {
+        throw new InvalidUserDataException("Groovy $variant is not compatible with Java $javaVersion")
+      }
+      javaVersion = 17
     }
   } else {
     throw new InvalidUserDataException("Unknown variant: $variant. Choose one of: $variants")