matrix-zulip-bridge: relax mautrix upper bound

The pyproject.toml pins mautrix>=0.20,<0.21, but nixpkgs ships
mautrix 0.21.0. The cap is conservative; the package works fine
against 0.21. Use pythonRelaxDeps to lift the bound.

Fixes pythonRuntimeDepsCheckHook failure:
  mautrix<0.21,>=0.20 not satisfied by version 0.21.0

ZHF: #516381

pkgs/by-name/ma/matrix-zulip-bridge/package.nix

@@ -36,6 +36,7 @@ python3Packages.buildPythonApplication (finalAttrs: {
   pythonRelaxDeps = [
     "bidict"
     "markdownify"
+    "mautrix"
     "ruamel-yaml"
     "zulip-emoji-mapping"
   ];