Re-enable BWC tests after backport of #53338 (#53389)

Author: DaveCTurner

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/53338" /* 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/coordination/JoinRequest.java

@@ -38,7 +38,7 @@ public class JoinRequest extends TransportRequest {
     /**
      * The minimum term for which the joining node will accept any cluster state publications. If the joining node is in a strictly greater
      * term than the master it wants to join then the master must enter a new term and hold another election. Doesn't necessarily match
-     * {@link JoinRequest#optionalJoin} and may be zero in join requests sent prior to {@link Version#V_8_0_0}.
+     * {@link JoinRequest#optionalJoin}.
      */
     private final long minimumTerm;
 
@@ -60,7 +60,7 @@ public JoinRequest(DiscoveryNode sourceNode, long minimumTerm, Optional<Join> op
     public JoinRequest(StreamInput in) throws IOException {
         super(in);
         sourceNode = new DiscoveryNode(in);
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
             minimumTerm = in.readLong();
         } else {
             minimumTerm = 0L;
@@ -72,7 +72,7 @@ public JoinRequest(StreamInput in) throws IOException {
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
         sourceNode.writeTo(out);
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
             out.writeLong(minimumTerm);
         }
         out.writeOptionalWriteable(optionalJoin.orElse(null));