Remove elasticsearch-java 7.15.x support

7.15.x (Sep 2021) had unstable API with different package structure
(_core vs core, base.rest_client vs transport.rest_client). The API
stabilized from 7.16.0+. Plugin now covers 7.16.x-9.x.

All remaining versions verified locally.

Author: wu-sheng

.github/workflows/plugins-test.2.yaml

@@ -98,7 +98,6 @@ jobs:
           - rocketmq-scenario
           - rocketmq-5-grpc-scenario
           - elasticsearch-java-scenario
-          - elasticsearch-java-7.15.x-scenario
     steps:
       - uses: actions/checkout@v2
         with:

apm-sniffer/apm-sdk-plugin/elasticsearch-java-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/java/define/RestClientTransportV1Instrumentation.java

@@ -14,7 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# Elasticsearch Java client (co.elastic.clients:elasticsearch-java) 7.15.x-9.x
-elasticsearch-java=org.apache.skywalking.apm.plugin.elasticsearch.java.define.RestClientTransportV1Instrumentation
+# Elasticsearch Java client (co.elastic.clients:elasticsearch-java) 7.16.x-9.x
 elasticsearch-java=org.apache.skywalking.apm.plugin.elasticsearch.java.define.RestClientTransportInstrumentation
 elasticsearch-java=org.apache.skywalking.apm.plugin.elasticsearch.java.define.ElasticsearchTransportBaseInstrumentation