Fix gRPC channel shutdown error

[antas-marcin]

This PR fixes this error that started occurring in tests:

2023-11-14 13:17:49 5.15.0 SEVERE *~*~*~ Previous channel ManagedChannelImpl{logId=1, target=localhost:32774} was not shutdown properly!!! ~*~*~*
    Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true. 
java.lang.RuntimeException: ManagedChannel allocation site
	at io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(ManagedChannelOrphanWrapper.java:102)
	at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:60)
	at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:51)
	at io.grpc.internal.ManagedChannelImplBuilder.build(ManagedChannelImplBuilder.java:655)
	at io.grpc.ForwardingChannelBuilder2.build(ForwardingChannelBuilder2.java:261)
	at io.weaviate.client.base.grpc.GrpcClient.create(GrpcClient.java:30)
	at io.weaviate.client.WeaviateClient.<init>(WeaviateClient.java:46)
	at io.weaviate.client.WeaviateClient.<init>(WeaviateClient.java:33)
	at io.weaviate.integration.client.batch.ClientBatchGrpcCreateTest.createClient(ClientBatchGrpcCreateTest.java:204)
	at io.weaviate.integration.client.batch.ClientBatchGrpcCreateTest.testCreateBatch(ClientBatchGrpcCreateTest.java:172)
	at io.weaviate.integration.client.batch.ClientBatchGrpcCreateTest.shouldCreateBatchUsingGRPC(ClientBatchGrpcCreateTest.java:47)