Update version for serialization of allowed system indices

This commit updates the serialization for allowed system indices after
the backport of the work and also re-enables bwc compatibility tests.

Relates elastic#52385
Relates elastic#53035
Relates elastic#53062

Author: jaymode

build.gradle

@@ -219,8 +219,8 @@ 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/52385" /* 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/common/util/concurrent/ThreadContext.java

@@ -272,7 +272,7 @@ public static Tuple<Map<String, String>, Map<String, Set<String>>> readHeadersFr
     }
 
     public static List<String> readAllowedSystemIndices(StreamInput in) throws IOException {
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // TODO update version on backport
+        if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
             return in.readOptionalStringList();
         } else {
             return emptyList();
@@ -663,7 +663,7 @@ private void writeTo(StreamOutput out, Map<String, String> defaultHeaders) throw
             }
 
             out.writeMap(responseHeaders, StreamOutput::writeString, StreamOutput::writeStringCollection);
-            if (out.getVersion().onOrAfter(Version.V_8_0_0)) { // TODO update version on backport
+            if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
                 out.writeOptionalStringCollection(allowedSystemIndexPatterns);
             }
         }