[FLINK-10781][fs] Update BucketingSinkMigrationTest for Flink 1.7

[hequn8128]

What is the purpose of the change

Update BucketingSinkMigrationTest for Flink 1.7

Brief change log

• Support restore from snapshots for Flink 1.7.
  • I followed the comments in the ITTest about how to generate snapshots.

Verifying this change

This change is already covered by existing tests, such as BucketingSinkMigrationTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[hequn8128]

The latest PR includes snapshots generated from release-1.7.0