Add check preventing the build with Groovy 2.5 and Java 17+ ...

this prevents an unintuitive error message in the Spock build

build.gradle

@@ -15,6 +15,8 @@ ext {
   baseVersion = "2.2"
   snapshotVersion = true
   milestone = 0
+  javaVersions = [8, 11, 17] // ensure that latest version is actually build on travis, otherwise no docs get published
+  javaVersion = (System.getProperty("javaVersion") ?: 8) as int
   variants = [2.5, 3.0, 4.0]
   variant = System.getProperty("variant") as BigDecimal ?: variants.first()
   buildScan.tag "groovy-$variant"
@@ -23,6 +25,9 @@ ext {
       groovyVersion = "2.5.15"
       minGroovyVersion = "2.5.0"
       maxGroovyVersion = "2.9.99"
+      if(javaVersion >= 17) {
+        throw new InvalidUserDataException("Groovy $variant is not compatible with Java $javaVersion")
+      }
     } else if (variant == 3.0) {
       groovyGroup = "org.codehaus.groovy"
       groovyVersion = "3.0.9"
@@ -51,8 +56,6 @@ ext {
   }
   fullVersion = baseVersion + ((!snapshotVersion && milestone) ? "-M$milestone" : "") + "-groovy-$variant" + (snapshotVersion ? "-SNAPSHOT" : '')
   variantLessVersion = baseVersion + (snapshotVersion ? "-SNAPSHOT" : (milestone ? "-M$milestone" : ""))
-  javaVersions = [8, 11, 17] // ensure that latest version is actually build on travis, otherwise no docs get published
-  javaVersion = (System.getProperty("javaVersion") ?: 8) as int
   buildScan.tag "Java $javaVersion"
 
   libs = [