Adjusting persistent task local abort supported version after backport (#75067)

This change adjusts the supported version added in #74115
following the backport to 7.15.0 that was done in #75002.

It is possible to remove far more code from master following
the backport, however, this would make the ML change to use
the functionality harder as then that would need substantial
changes during its backport.  It will be easier to make the
ML change that uses the functionality while master still has
the concept of supported/not supported for this functionality,
then do a final followup PR that removes the unnecessary code
from master.

build.gradle

@@ -144,9 +144,9 @@ 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/75067"
+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/persistent/CompletionPersistentTaskAction.java

@@ -42,7 +42,7 @@ public class CompletionPersistentTaskAction extends ActionType<PersistentTaskRes
     public static final CompletionPersistentTaskAction INSTANCE = new CompletionPersistentTaskAction();
     public static final String NAME = "cluster:admin/persistent/completion";
 
-    public static final Version LOCAL_ABORT_AVAILABLE_VERSION = Version.V_8_0_0;
+    public static final Version LOCAL_ABORT_AVAILABLE_VERSION = Version.V_7_15_0;
 
     private CompletionPersistentTaskAction() {
         super(NAME, PersistentTaskResponse::new);

server/src/test/java/org/elasticsearch/persistent/CompletionPersistentTaskRequestTests.java

@@ -39,6 +39,6 @@ public class CompletionPersistentTaskRequestTests extends AbstractWireSerializin
         when(out.getVersion()).thenReturn(Version.V_7_14_0);
         IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> request.writeTo(out));
         assertThat(e.getMessage(), equalTo("attempt to abort a persistent task locally in a cluster that contains a node that is too "
-            + "old: found node version [7.14.0], minimum required [8.0.0]"));
+            + "old: found node version [7.14.0], minimum required [7.15.0]"));
     }
 }