SLING-12056 build by default with JDK21 as well (#20)

(next to 11 and 17)
diff --git a/vars/slingOsgiBundleBuild.groovy b/vars/slingOsgiBundleBuild.groovy
index 51187bf..2dd5540 100644
--- a/vars/slingOsgiBundleBuild.groovy
+++ b/vars/slingOsgiBundleBuild.groovy
@@ -3,19 +3,21 @@
 def call(Map params = [:]) {
 
     def globalConfig = [
+        // https://cwiki.apache.org/confluence/x/kRLiAw (most are Eclipse Temurin distributions of Open JDK from Adoptium)
         availableJDKs : [ 8: 'jdk_1.8_latest', 9: 'jdk_1.9_latest', 10: 'jdk_10_latest', 11: 'jdk_11_latest',
                           12: 'jdk_12_latest', 13: 'jdk_13_latest', 14: 'jdk_14_latest', 15: 'jdk_15_latest',
                           16: 'jdk_16_latest', 17: 'jdk_17_latest', 18: 'jdk_18_latest', 19: 'jdk_19_latest', 
                           20: 'jdk_20_latest', 21: 'jdk_21_latest', 22: 'jdk_22_latest'],
+        // https://cwiki.apache.org/confluence/x/cRTiAw
         mvnVersion : 'maven_3_latest',
-        // maps values to node labels (available ones in https://cwiki.apache.org/confluence/display/INFRA/ci-builds.apache.org)
+        // maps values to node labels (available ones in https://cwiki.apache.org/confluence/x/ViZ4CQ)
         availableOperatingSystems : ['windows' : 'Windows', 'linux': 'ubuntu', 'linux-arm': 'arm', 'ubuntu': 'ubuntu'],
         mainNodeLabel : 'ubuntu',
         githubCredentialsId: 'sling-github-token'
     ]
 
     def jobConfig = [
-        jdks: [11,17],
+        jdks: [11,17,21],
         operatingSystems: ['linux','windows'],
         upstreamProjects: [],
         archivePatterns: [],