Force renovatebot to run with java 21 (#4426)

Author: janhoy

.github/renovate.json

@@ -9,6 +9,7 @@
   "includePaths": ["gradle/libs.versions.toml", "versions.*", "build.gradle", ".github/workflows/*"],
   "postUpgradeTasks": {
     "commands": [
+      "java -version 2>&1 | grep -q '21\\.' || install-tool java 21.0.11+10.0.LTS",
       "./gradlew resolveAndLockAll --write-locks",
       "./gradlew kotlinUpgradeYarnLock",
       "./gradlew updateLicenses"