chore: improve embedded version handling

Use new feature in releasetool (googleapis/releasetool#317) to manage the client version instead of maven tricks introduced in googleapis#451

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/ClientVersion.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.cloud.bigtable;
+
+import com.google.api.core.InternalApi;
+
+@InternalApi("For internal use only")
+public final class ClientVersion {
+  // {x-version-update-start:google-cloud-bigtable:current}
+  public static String VERSION = "1.22.1-SNAPSHOT";
+  // {x-version-update-end}
+}