nixos/matrix-synapse: drop old DB check assertion, actually require DB to be up

[Ma27]

Description of changes

Closes #236062

The PR #236062 was submitted because of the following problem: a synapse instance was running in a NixOS container attached to the host network and a postgresql instance on the host as database. In this setup, synapse connected to its DB via 127.0.0.1, but the DB wasn't locally set up and thus not configured in NixOS (i.e.
config.services.postgresql.enable was false). This caused the assertion removed in this patch to fail.

Over three years ago this assertion was introduced when this module stopped doing autoconfiguration of postgresql entirely[1] because a breaking change in synapse couldn't be managed via an auto-upgrade on our side. To make sure people don't deploy their DB away by accident, this assertion was introduced.

Nowadays this doesn't serve any value anymore because people with existing instances should've upgraded by now (otherwise it's their job to carefully read the release notes when missing upgrades for several years) and people deploying fresh instances are instructed by the docs to also configure postgresql[2].

Instead, it only causes issues in corner cases like #236062, so after some discussion in that PR I think it's time to remove the assertion altogether.

Also, there's no Requires= for postgresql.service in the systemd units which means that it's not strictly guaranteed that the DB is up when synapse starts up. This is fixed now by adding requires. To avoid being bitten by above mentioned cases again, this only happens if config.services.postgresql.enable is true.

If somebody uses a non-local postgresql, but has also deployed a local postgresql instance on the synapse server (rather unlikely IMHO), it's their job to opt out of this behavior with mkForce (this is precisely one of the use-cases mkForce and friends were built for IMHO).

[1] #80447
[2] https://nixos.org/manual/nixos/stable/#module-services-matrix-synapse

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.