Reenable BWC after backport of #75606 (#76617)

This PR changes the serialization version for the contents of #75606 and re-enables BWC tests following the backport of that PR (backport in #76587).

build.gradle

@@ -138,9 +138,9 @@ tasks.register("verifyVersions") {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = false
+boolean bwc_tests_enabled = true
 // place a PR link here when committing bwc changes:
-String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/76587"
+String bwc_tests_disabled_issue = ""
 /*
  * 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

server/src/main/java/org/elasticsearch/cluster/routing/UnassignedInfo.java

@@ -47,7 +47,7 @@ public final class UnassignedInfo implements ToXContentFragment, Writeable {
      * The version that the {@code lastAllocatedNode} field was added in. Used to adapt streaming of this class as appropriate for the
      * version of the node sending/receiving it. Should be removed once wire compatibility with this version is no longer necessary.
      */
-    private static final Version VERSION_LAST_ALLOCATED_NODE_ADDED = Version.V_8_0_0;
+    private static final Version VERSION_LAST_ALLOCATED_NODE_ADDED = Version.V_7_15_0;
 
     public static final DateFormatter DATE_TIME_FORMATTER = DateFormatter.forPattern("date_optional_time").withZone(ZoneOffset.UTC);