fix: uses old version of gax-grpc method

Internally, we are using version 1.47.1 of the gax-grpc library, which
does not provide a method currently being used (setExecutor).

In here we use a method that is available in 1.47.1 and achieves the
same goal (setExecutorProvider).

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java

@@ -60,7 +60,6 @@
 import com.google.cloud.spanner.admin.instance.v1.stub.GrpcInstanceAdminStub;
 import com.google.cloud.spanner.admin.instance.v1.stub.InstanceAdminStub;
 import com.google.cloud.spanner.admin.instance.v1.stub.InstanceAdminStubSettings;
-import com.google.cloud.spanner.spi.v1.SpannerRpc.Option;
 import com.google.cloud.spanner.v1.stub.GrpcSpannerStub;
 import com.google.cloud.spanner.v1.stub.SpannerStub;
 import com.google.cloud.spanner.v1.stub.SpannerStubSettings;
@@ -317,7 +316,11 @@ public GapicSpannerRpc(final SpannerOptions options) {
             .setMaxInboundMessageSize(MAX_MESSAGE_SIZE)
             .setMaxInboundMetadataSize(MAX_METADATA_SIZE)
             .setPoolSize(options.getNumChannels())
-            .setExecutor(executorProvider.getExecutor())
+
+            // Before updating this method to setExecutor, please make sure to verify the version
+            // of gax-grpc used internally. At the moment we are using 1.47.1, which does not
+            // support the setExecutor variant.
+            .setExecutorProvider(executorProvider)
 
             // Set a keepalive time of 120 seconds to help long running
             // commit GRPC calls succeed