Update docs to specify utf8mb4 default (#11962)

Install tool already sets it as the default so we can just update
example ini and docs to it.

Fixes: #11081

Author: silverwind

custom/conf/app.example.ini

@@ -375,9 +375,9 @@ SCHEMA =
 ; For Postgres, either "disable" (default), "require", or "verify-full"
 ; For MySQL, either "false" (default), "true", or "skip-verify"
 SSL_MODE = disable
-; For MySQL only, either "utf8" or "utf8mb4", default is "utf8".
+; For MySQL only, either "utf8" or "utf8mb4", default is "utf8mb4".
 ; NOTICE: for "utf8mb4" you must use MySQL InnoDB > 5.6. Gitea is unable to check this.
-CHARSET = utf8
+CHARSET = utf8mb4
 ; For "sqlite3" and "tidb", use an absolute path when you start gitea as service
 PATH = data/gitea.db
 ; For "sqlite3" only. Query timeout

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

@@ -245,7 +245,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
      - `require`: Enable TLS without any verifications.
      - `verify-ca`: Enable TLS with verification of the database server certificate against its root certificate.
      - `verify-full`: Enable TLS and verify the database server name matches the given certificate in either the `Common Name` or `Subject Alternative Name` fields.
-- `CHARSET`: **utf8**: For MySQL only, either "utf8" or "utf8mb4", default is "utf8". NOTICE: for "utf8mb4" you must use MySQL InnoDB > 5.6. Gitea is unable to check this.
+- `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/advanced/config-cheat-sheet.zh-cn.md

@@ -81,7 +81,7 @@ menu:
 - `USER`: 数据库用户名。
 - `PASSWD`: 数据库用户密码。
 - `SSL_MODE`: MySQL 或 PostgreSQL数据库是否启用SSL模式。
-- `CHARSET`: **utf8**: 仅当数据库为 MySQL 时有效, 可以为 "utf8" 或 "utf8mb4"。注意：如果使用 "utf8mb4"，你的 MySQL InnoDB 版本必须在 5.6 以上。
+- `CHARSET`: **utf8mb4**: 仅当数据库为 MySQL 时有效, 可以为 "utf8" 或 "utf8mb4"。注意：如果使用 "utf8mb4"，你的 MySQL InnoDB 版本必须在 5.6 以上。
 - `PATH`: Tidb 或者 SQLite3 数据文件存放路径。
 - `LOG_SQL`: **true**: 显示生成的SQL，默认为真。
 - `MAX_IDLE_CONNS` **0**: 最大空闲数据库连接