Adjust IndexMetadata serialization version (#60988)

This change adjusts the IndexMetadata serialization version for the
system index flag after the backport of #57936. Additionally, bwc tests
have been reenabled.

build.gradle

@@ -174,8 +174,8 @@ tasks.register("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/57936" /* 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/cluster/metadata/IndexMetadata.java

@@ -345,7 +345,7 @@ public class IndexMetadata implements Diffable<IndexMetadata>, ToXContentFragmen
 
     public static final String INDEX_STATE_FILE_PREFIX = "state-";
 
-    static final Version SYSTEM_INDEX_FLAG_ADDED = Version.V_8_0_0;
+    static final Version SYSTEM_INDEX_FLAG_ADDED = Version.V_7_10_0;
 
     private final int routingNumShards;
     private final int routingFactor;