[FLINK-10798] Add the version number of Flink 1.7 to MigrationVersion

[yanghua]

What is the purpose of the change

Add the version number of Flink 1.7 to MigrationVersion

Brief change log

• Add the version number of Flink 1.7 to MigrationVersion

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[yanghua]

@tzulitai @zentol

[zentol]

I would not merge this right now, as it encourages people to update tests now when this should only be done once 1.7 is released.

[tzulitai]

Agreed, we should only merge this after 1.7 is released.

[yanghua]

Since Flink 1.7 has released, shall we merge this PR?

[yanghua]

cc @tillrohrmann

[tzulitai]

+1 LGTM, will merge this now.