CCR follower index needs to copy transport version from CCR leader index (elastic#145035)

Currently, when adding a follower index to a leader 
index using CCR, transport version is not copied which 
leads to errors in the upgrade assistant.

Author: jdconrad

docs/changelog/145035.yaml

@@ -0,0 +1,5 @@
+area: CCR
+issues: []
+pr: 145035
+summary: CCR follower index needs to copy transport version from CCR leader index
+type: bug

x-pack/plugin/ccr/src/internalClusterTest/java/org/elasticsearch/xpack/ccr/CcrRepositoryIT.java

@@ -199,6 +199,7 @@ public void testThatRepositoryRecoversEmptyIndexBasedOnLeaderSettings() throws I
 
         // UUID is changed so that we can follow indexes on same cluster
         assertNotEquals(leaderMetadata.getIndexUUID(), followerMetadata.getIndexUUID());
+        assertEquals(leaderMetadata.getTransportVersion(), followerMetadata.getTransportVersion());
     }
 
     public void testDocsAreRecovered() throws Exception {

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/repository/CcrRepository.java

@@ -309,6 +309,7 @@ public IndexMetadata getSnapshotIndexMetaData(RepositoryData repositoryData, Sna
         imdBuilder.putCustom(Ccr.CCR_CUSTOM_METADATA_KEY, customMetadata);
 
         imdBuilder.settings(leaderIndexMetadata.getSettings());
+        imdBuilder.transportVersion(leaderIndexMetadata.getTransportVersion());
 
         // Copy mappings from leader IMD to follow IMD
         imdBuilder.putMapping(leaderIndexMetadata.mapping());