nixos/redmine: Fix database assertions

Recent PR 266270[1] modified an assertion related to database settings
of the redmine service. There are two problems with that change:

1. Assert message was not updated to reflect the change in the assert
   condition.

2. The new condition applies only to postgresql, not the default
   mysql. Therefore, the assertion breaks existing mysql-based
   installations without any reason.

This commit fixes these by 1) reverting the modified assertion to the
previous value, making the message match the condition and 2) adding a
new assertion that applies only to postgresql.

[1]: https://github.com/NixOS/nixpkgs/pull/266270

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