Remove "CHARSET" config option for MySQL, always use "utf8mb4" (#25413)

In modern days, there is no reason to make users set "charset" anymore.

Close #25378

## ⚠️ BREAKING

The key `[database].CHARSET` was removed completely as every newer
(>10years) MySQL database supports `utf8mb4` already.
There is a (deliberately) undocumented new fallback option if anyone
still needs to use it, but we don't recommend using it as it simply
causes problems.

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

@@ -443,7 +443,6 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `SQLITE_TIMEOUT`: **500**: Query timeout for SQLite3 only.
 - `SQLITE_JOURNAL_MODE`: **""**: Change journal mode for SQlite3. Can be used to enable [WAL mode](https://www.sqlite.org/wal.html) when high load causes write congestion. See [SQlite3 docs](https://www.sqlite.org/pragma.html#pragma_journal_mode) for possible values. Defaults to the default for the database file, often DELETE.
 - `ITERATE_BUFFER_SIZE`: **50**: Internal buffer size for iterating.
-- `CHARSET`: **utf8mb4**: For MySQL only, either "utf8" or "utf8mb4". NOTICE: for "utf8mb4" you must use MySQL InnoDB > 5.6. Gitea is unable to check this.
 - `PATH`: **data/gitea.db**: For SQLite3 only, the database file path.
 - `LOG_SQL`: **true**: Log the executed SQL.
 - `DB_RETRIES`: **10**: How many ORM init / DB connect attempts allowed.

docs/content/doc/help/faq.en-us.md

@@ -396,8 +396,6 @@ Please run `gitea convert`, or run `ALTER DATABASE database_name CHARACTER SET u
 for the database_name and run `ALTER TABLE table_name CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;`
 for each table in the database.
 
-You will also need to change the app.ini database charset to `CHARSET=utf8mb4`.
-
 ## Why are Emoji displaying only as placeholders or in monochrome
 
 Gitea requires the system or browser to have one of the supported Emoji fonts installed, which are Apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol, Noto Color Emoji and Twemoji Mozilla. Generally, the operating system should already provide one of these fonts, but especially on Linux, it may be necessary to install them manually.