Remove mysql and mariadb chart deps (#417)

As discussed in Discord.

Supersedes #412 and #407.

**⚠️ BREAKING**

Users depending on the built-in MySQL or MariaDB chart have to switch to an self-managed database, or Postgres

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/417
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.io>
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>
Reviewed-by: yardenshoham <yardenshoham@noreply.gitea.io>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

.markdownlint.yaml

@@ -131,8 +131,6 @@ MD044:
   names:
   - Gitea
   - PostgreSQL
-  - MariaDB
-  - MySQL
   - Memcached
   - Prometheus
   - Git