feat: support django v4.2.1

[wd0517]

Feature

• Add column_comment when introspecting table schema

Fix

• TiDB doesn't support defaults for BLOB/TEXT/JSON in the ALTER COLUMN statement.
• Django 4.2.1 no longer supports MySQL 5.7, we need to explicitly declare the tidb_version in get_database_version
• Remove hard code in tidb_settings.py

[sre-bot]

All committers have signed the CLA.

[wd0517]

Would be good to update .github/workflows/ci.yaml to also test with v4.2

I will create a new pr to update ci, not only django v4.2, also add tidb v6 & v7, follow the tidb release support policy