Recommend/convert to use case-sensitive collation for MySQL/MSSQL (#28662)

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.

docs/content/administration/config-cheat-sheet.en-us.md

@@ -430,6 +430,7 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `NAME`: **gitea**: Database name.
 - `USER`: **root**: Database username.
 - `PASSWD`: **_empty_**: Database user password. Use \`your password\` or """your password""" for quoting if you use special characters in the password.
+- `CHARSET_COLLATION`: **_empty_**: (MySQL/MSSQL only) Gitea expects to use a case-sensitive collation for database. Leave it empty to use the default collation decided by the Gitea. Don't change it unless you clearly know what you need.
 - `SCHEMA`: **_empty_**: For PostgreSQL only, schema to use if different from "public". The schema must exist beforehand,
   the user must have creation privileges on it, and the user search path must be set to the look into the schema first
   (e.g. `ALTER USER user SET SEARCH_PATH = schema_name,"$user",public;`).