[Transform] reenable bwc (#53329)

re-enable BWC and finish backport of #53324

Author: Hendrik Muhs

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/53324" /* 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")

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/transform/transforms/pivot/SingleGroupSource.java

@@ -80,7 +80,7 @@ public SingleGroupSource(final String field, final ScriptConfig scriptConfig) {
 
     public SingleGroupSource(StreamInput in) throws IOException {
         field = in.readOptionalString();
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // change to 7.7.0
+        if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
             scriptConfig = in.readOptionalWriteable(ScriptConfig::new);
         } else {
             scriptConfig = null;
@@ -107,7 +107,7 @@ protected void innerXContent(XContentBuilder builder, Params params) throws IOEx
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         out.writeOptionalString(field);
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
             out.writeOptionalWriteable(scriptConfig);
         }
     }