Use ".RELEASE" suffix for releases on 5.2.x branch

This commit ensures that the release scripts generate a ".RELEASE"
suffixed release version, since the 5.2.x branch is still using this
naming scheme.

See https://github.com/spring-io/concourse-java-scripts#get_next_release
See spring-projectsgh-26659

ci/scripts/stage-version.sh

@@ -20,7 +20,7 @@ elif [[ $RELEASE_TYPE = "RC" ]]; then
 	stageVersion=$( get_next_rc_release $snapshotVersion)
 	nextVersion=$snapshotVersion
 elif [[ $RELEASE_TYPE = "RELEASE" ]]; then
-	stageVersion=$( get_next_release $snapshotVersion)
+	stageVersion=$( get_next_release $snapshotVersion "RELEASE")
 	nextVersion=$( bump_version_number $snapshotVersion)
 else
 	echo "Unknown release type $RELEASE_TYPE" >&2; exit 1;