[MM-63314] Fix ClaimJob in HA environments (#30383)

* ClaimJob now returns newly claimed job

* internal code affected by change

* test changes required

* two branches: for mysql, use transaction; for postgres, use returning

* two branches: for mysql, use transaction; for postgres, use returning

* use same millis value for LastActivityAt and StartAt

* blank commit

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/enterprise/elasticsearch/opensearch/aggregation_job_test.go

@@ -60,7 +60,8 @@ func TestElasticsearchAggregation(t *testing.T) {
 	mockJobStore.On("UpdateStatusOptimistically",
 		mock.AnythingOfType("string"),
 		model.JobStatusPending,
-		model.JobStatusInProgress).Return(true, nil)
+		model.JobStatusInProgress).
+		Return(&model.Job{}, nil)
 	mockJobStore.On("GetAllByType", mock.AnythingOfType("string")).Return([]*model.Job{{
 		Id:     "abcxyz123",
 		Type:   "EnterpriseElasticsearchIndexer",