chore(deps): upgrade gapic-generator-java to 2.8.0 and update gax-java to 2.18.1 (#1367)

- [ ] 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]

google-cloud-bigquerydatatransfer/src/main/java/com/google/cloud/bigquery/datatransfer/v1/DataTransferServiceClient.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.paging.AbstractFixedSizeCollection;
 import com.google.api.gax.paging.AbstractPage;
@@ -128,7 +127,6 @@ public static final DataTransferServiceClient create(DataTransferServiceSettings
    * Constructs an instance of DataTransferServiceClient, using the given stub for making calls.
    * This is for advanced usage - prefer using create(DataTransferServiceSettings).
    */
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public static final DataTransferServiceClient create(DataTransferServiceStub stub) {
     return new DataTransferServiceClient(stub);
   }
@@ -143,7 +141,6 @@ protected DataTransferServiceClient(DataTransferServiceSettings settings) throws
     this.stub = ((DataTransferServiceStubSettings) settings.getStubSettings()).createStub();
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   protected DataTransferServiceClient(DataTransferServiceStub stub) {
     this.settings = null;
     this.stub = stub;
@@ -153,7 +150,6 @@ public final DataTransferServiceSettings getSettings() {
     return settings;
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public DataTransferServiceStub getStub() {
     return stub;
   }

google-cloud-bigquerydatatransfer/src/main/java/com/google/cloud/bigquery/datatransfer/v1/stub/DataTransferServiceStubSettings.java

@@ -479,7 +479,6 @@ public UnaryCallSettings<EnrollDataSourcesRequest, Empty> enrollDataSourcesSetti
     return enrollDataSourcesSettings;
   }
 
-  @BetaApi("A restructuring of stub classes is planned, so this may break in the future")
   public DataTransferServiceStub createStub() throws IOException {
     if (getTransportChannelProvider()
         .getTransportName()