support django 5.0

[wd0517]

Django 5.0 New Feature (db related):

1. Database-computed default values, TiDB has limited support for default value expressions, see docs.
2. Database generated model field, TiDB also has some limitations.

This PR also dropped support for TiDB 4.x as it has already reached its EOL.

There are two TiDB bugs discovered by the Django test suite that need to be tracked:

• Failed to modify column's default value when has expression index tidb#52355
• Wrong referenced_table_schema in information_schema.key_column_usage tidb#52350

[dveeden]

@wd0517 the description says "This PR also dropped support for TiDB 5.x as it has already reached its EOL." But this doesn't look to match with the changes. Isn't this dropping support for TiDB 4.x instead?

[wd0517]

@wd0517 the description says "This PR also dropped support for TiDB 5.x as it has already reached its EOL." But this doesn't look to match with the changes. Isn't this dropping support for TiDB 4.x instead?

Sorry, it was a typo, drop support for TiDB 4.x