Reenable BwC Tests after #53372 (#53381)

Reenabling BwC Tests now that #53372 has been merged.

Author: original-brownbear

build.gradle

@@ -220,8 +220,8 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = false
-final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/53372" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = true
+final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")

server/src/main/java/org/elasticsearch/action/admin/indices/forcemerge/ForceMergeRequest.java

@@ -56,7 +56,7 @@ public static final class Defaults {
     private boolean onlyExpungeDeletes = Defaults.ONLY_EXPUNGE_DELETES;
     private boolean flush = Defaults.FLUSH;
 
-    private static final Version FORCE_MERGE_UUID_VERSION = Version.V_8_0_0;
+    private static final Version FORCE_MERGE_UUID_VERSION = Version.V_7_7_0;
 
     /**
      * Force merge UUID to store in the live commit data of a shard under