Reenable bwc tests for #76475 (#76576)

Reenabling bwc tests after merge of #76475

Author: imotov

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/76475"
+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/search/aggregations/bucket/terms/InternalMappedTerms.java

@@ -52,7 +52,7 @@ protected InternalMappedTerms(String name, BucketOrder reduceOrder, BucketOrder
      */
     protected InternalMappedTerms(StreamInput in, Bucket.Reader<B> bucketReader) throws IOException {
         super(in);
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // todo fix after backport
+        if (in.getVersion().onOrAfter(Version.V_7_15_0)) {
             if (in.readBoolean()) {
                 docCountError = in.readZLong();
             } else {
@@ -70,7 +70,7 @@ protected InternalMappedTerms(StreamInput in, Bucket.Reader<B> bucketReader) thr
 
     @Override
     protected final void writeTermTypeInfoTo(StreamOutput out) throws IOException {
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {  // todo fix after backport
+        if (out.getVersion().onOrAfter(Version.V_7_15_0)) {
             if (docCountError != null) {
                 out.writeBoolean(true);
                 out.writeZLong(docCountError);