Backport #24105 by @lunny
In #12269, all string fields of struct will generate a NVARCHAR column
in database, but for those Gitea instances installed before that PR,
users have to convert columns themselves.
In this PR, we update the `./gitea admin convert` commands to support
both MySQL and MSSQL database converting. Previously, it only supported
converting `utf8 -> utf8mb4` for MySQL.
Now, it will check the database types.
If it's MSSQL, it will convert `VARCHAR -> NVARCHAR` as well.
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
c4f569b9a5