Prepare backport of #51260. (#51876)

Backport: #51875

Author: jpountz

build.gradle

@@ -215,8 +215,8 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = true
-final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = false
+final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/51875" /* 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")

modules/analysis-common/src/test/resources/rest-api-spec/test/cluster.stats/10_analysis_stats.yml

@@ -2,7 +2,7 @@
 "get cluster stats returns analysis stats":
 
   - skip:
-      version: " - 7.99.99" # TODO: 7.6.99
+      version: " - 7.6.99"
       reason:  "analysis stats are added for v7.7.0"
 
   - do:

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.stats/10_basic.yml

@@ -88,7 +88,7 @@
 "get cluster stats returns mapping stats":
 
   - skip:
-      version: " - 7.99.99" # TODO: 7.6.99
+      version: " - 7.6.99"
       reason:  "mapping stats are added for v7.7.0"
 
   - do:

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsResponse.java

@@ -53,7 +53,7 @@ public ClusterStatsResponse(StreamInput in) throws IOException {
         String clusterUUID = null;
         MappingStats mappingStats = null;
         AnalysisStats analysisStats = null;
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // TODO: 7_7_0
+        if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
             clusterUUID = in.readOptionalString();
             mappingStats = in.readOptionalWriteable(MappingStats::new);
             analysisStats = in.readOptionalWriteable(AnalysisStats::new);
@@ -112,7 +112,7 @@ public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
         out.writeVLong(timestamp);
         out.writeOptionalWriteable(status);
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
             out.writeOptionalString(clusterUUID);
             out.writeOptionalWriteable(indicesStats.getMappings());
             out.writeOptionalWriteable(indicesStats.getAnalysis());