Library Compatibility with Protobuf-Java 4.26.x+ #1690

Protobuf has released v26.x+ which introduces a bunch of breaking changes for Protobuf-Java. Release notes for the version can be found at https://github.com/protocolbuffers/protobuf/releases/tag/v26.0.

We have identified references to GeneratedMessageV3 which no longer exists in these test files:

1. java-firestore/google-cloud-firestore/src/test/java/com/google/cloud/firestore/LocalFirestoreHelper.java

   Line 1203 in 17fdb4e

   GeneratedMessageV3 request;

2. java-firestore/google-cloud-firestore/src/test/java/com/google/cloud/firestore/BulkWriterTest.java

   Line 77 in 17fdb4e

   public static final ApiFuture<GeneratedMessageV3> FAILED_FUTURE =

3. java-firestore/google-cloud-firestore/src/test/java/com/google/cloud/firestore/TransactionTest.java

   Line 91 in 17fdb4e

   private final ApiFuture<GeneratedMessageV3> RETRYABLE_API_EXCEPTION =

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