[FLINK-27929][python] Drop support for python36

[bzhaoopenstack]

This PR will drop the py36 support for pyflink.

What is the purpose of the change

py36 had reached its EOL. https://endoflife.date/python So need to drop it from pyflink.

Brief change log

• drop system py36 version validation, increase to 3.7
• associate py36 doc change

Verifying this change

py36 won't be supported by pyflink 1.17

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, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

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

[flinkbot]

CI report:

• ae345fe71057b9e57257df61de907f05f224355b Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[dianfu]

Closed this PR as this should have already been supported.