CI - run tests on multiple databases

1 - Update https://github.com/BeaconCMS/beacon/blob/main/.github/workflows/elixir.yml to run tests on postgres, mysql, and mssql

2 - Update README.md to mention that Beacon supports the [3 built-in ecto adapters](https://hexdocs.pm/ecto_sql/Ecto.Adapters.SQL.html#module-built-in-adapters): postgres, mysql, mssql