Use utf8mb4 character set in MariaDB config

Since MariaDB 10.6 the 'utf8' character set is an alias for 'utf8mb3'.
This seems to trip up something and HedgeDoc can't connect to the database anymore.

This commit changes the character set to 'utf8mb4', which is the "real" UTF-8 charset for MariaDB.

References:
https://mariadb.com/kb/en/upgrading-from-mariadb-105-to-mariadb-106/#character-sets
https://mariadb.com/kb/en/mariadb-1061-release-notes/#character-sets
https://mathiasbynens.be/notes/mysql-utf8mb4Signed-off-by: David Mehren <git@herrmehren.de>

resources/utf8.cnf

 [client]
-default-character-set=utf8
+default-character-set=utf8mb4
 
 [mysql]
-default-character-set=utf8
-
+default-character-set=utf8mb4
 
 [mysqld]
-collation-server = utf8_unicode_ci
-init-connect='SET NAMES utf8'
-character-set-server = utf8
+collation-server = utf8mb4_unicode_ci
+init-connect='SET NAMES utf8mb4'
+character-set-server = utf8mb4