chore(deps): upgrade gapic-generator-java to 2.8.0 and update gax-java to 2.18.1 (#152)

- [ ] Regenerate this pull request now.

PiperOrigin-RevId: 450543911

Source-Link: googleapis/googleapis@5528344

Source-Link: https://github.com/googleapis/googleapis-gen/commit/9f6775cab1958982b88967a43e5e806af0f135db
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiOWY2Nzc1Y2FiMTk1ODk4MmI4ODk2N2E0M2U1ZTgwNmFmMGYxMzVkYiJ9

Author: gcf-owl-bot[bot]

java-storage-transfer/google-cloud-storage-transfer/src/main/java/com/google/storagetransfer/v1/proto/StorageTransferServiceClient.java

@@ -18,7 +18,6 @@
 
 import com.google.api.core.ApiFuture;
 import com.google.api.core.ApiFutures;
-import com.google.api.core.BetaApi;
 import com.google.api.gax.core.BackgroundResource;
 import com.google.api.gax.longrunning.OperationFuture;
 import com.google.api.gax.paging.AbstractFixedSizeCollection;
@@ -138,7 +137,6 @@ public static final StorageTransferServiceClient create(StorageTransferServiceSe
    * Constructs an instance of StorageTransferServiceClient, using the given stub for making calls.
    * This is for advanced usage - prefer using create(StorageTransferServiceSettings).
    */
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public static final StorageTransferServiceClient create(StorageTransferServiceStub stub) {
     return new StorageTransferServiceClient(stub);
   }
@@ -155,7 +153,6 @@ protected StorageTransferServiceClient(StorageTransferServiceSettings settings)
     this.operationsClient = OperationsClient.create(this.stub.getOperationsStub());
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   protected StorageTransferServiceClient(StorageTransferServiceStub stub) {
     this.settings = null;
     this.stub = stub;
@@ -166,7 +163,6 @@ public final StorageTransferServiceSettings getSettings() {
     return settings;
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public StorageTransferServiceStub getStub() {
     return stub;
   }

java-storage-transfer/google-cloud-storage-transfer/src/main/java/com/google/storagetransfer/v1/proto/stub/StorageTransferServiceStubSettings.java

@@ -384,7 +384,6 @@ public UnaryCallSettings<TransferProto.DeleteAgentPoolRequest, Empty> deleteAgen
     return deleteAgentPoolSettings;
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public StorageTransferServiceStub createStub() throws IOException {
     if (getTransportChannelProvider()
         .getTransportName()