Re-enable BWC tests after backport of ILM frozen phase changes

With elastic#70158 (master), elastic#70170 (7.x), and elastic#70171 (7.12) merged, we can now re-enable backwards
compatibilty tests on master and 7.x.

build.gradle

@@ -189,8 +189,8 @@ tasks.register("verifyVersions") {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = false
-String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/70158" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = true
+String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
 /*
  * FIPS 140-2 behavior was fixed in 7.11.0. Before that there is no way to run elasticsearch in a
  * JVM that is properly configured to be in fips mode with BCFIPS. For now we need to disable