Default to minimum required Java version for Groovy 6

Groovy 6 requires Java 17+. Following the same pattern introduced for
Groovy 5, build.gradle silently defaults to Java 17 when none was specified.
An explicit but incompatible javaVersion still fails loudly.

allVariants and allVariants.bat now include 6.0 in the loop, so
e.g. `allVariants -DjavaVersion=21` works correctly for all variants.

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")