correct charset for mariadb connector with mdb 10.6

correct charset for mariadb connector with mdb 10.6

Updated test suite to pass correctly for MariaDB 10.6 when using the
mariadb-connector driver, which made some adjustments to default encoding.
Pull request courtesy Georg Richter.

Use character set utf8mb4 instead of utf8:

1) Character set for mariadb connector is utf8mb4 and
cannot be changed.

2) Since MariaDB 10.6.1 utf8 is mapped to utf8mb3, and
will be mapped to utf8mb4 in 10.7, see [MDEV-8334](https://jira.mariadb.org/browse/MDEV-8334) (rename
utf8 to utf8mb3)

utf8mb4 character set support was added 12 years ago and
will work with all non eoled MariaDB server versions.

Fixes: #7115
Closes: #7116
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7116
Pull-request-sha: 79516ccce2dead11e6658596c9ad400b964d24c9

Change-Id: I86c30fb636c46307cb551080641e324d2bbc1cb3