Mainly for MySQL/MSSQL.
It is important for Gitea to use case-sensitive database charset
collation. If the database is using a case-insensitive collation, Gitea
will show startup error/warning messages, and show the errors/warnings
on the admin panel's Self-Check page.
Make `gitea doctor convert` work for MySQL to convert the collations of
database & tables & columns.
* Fix #28131
## ⚠️ BREAKING ⚠️
It is not quite breaking, but it's highly recommended to convert the
database&table&column to a consistent and case-sensitive collation.
Replace #21790
And close #25965 by the way (it needs a separate fix for 1.20)
Major changes:
1. Move "gitea convert" to "gitea doctor conver". The old "gitea doctor"
still works as a hidden sub-command (to avoid breaking)
2. Do not write "doctor.log" by default, it's not useful in most cases
and causes bugs like 25965
3. Improve documents
4. Fix the "help" commands. Before, the "./gitea doctor" can't show the
sub-command help correctly (regression of the last cli/v2 refactoring)
After this PR:
```
./gitea help # show all sub-commands for the app
./gitea doctor # show the sub-commands for the "doctor"
./gitea doctor help # show the sub-commands for the "doctor", as above
```
