Use uv to test full set of minimum deps in CI (#19289)

Stemming from #19274 this updates the `olddeps` CI to test against not
just the minimum version of our explicit dependencies, but also the
minimum version of all implicit (transitive) dependencies that are
pulled in from the explicit dependencies themselves.

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

.github/workflows/tests.yml

@@ -452,14 +452,12 @@ jobs:
           python-version: '3.10'
 
       - name: Prepare old deps
-        if: steps.cache-poetry-old-deps.outputs.cache-hit != 'true'
-        run: .ci/scripts/prepare_old_deps.sh
-
-      # Note: we install using `pip` here, not poetry. `poetry install` ignores the
-      # build-system section (https://github.com/python-poetry/poetry/issues/6154), but
-      # we explicitly want to test that you can `pip install` using the oldest version
-      # of poetry-core and setuptools-rust.
-      - run: pip install .[all,test]
+        # Note: we install using `uv` here, not poetry or pip to allow us to test with the
+        # minimum version of all dependencies, both those explicitly specified and those
+        # implicitly brought in by the explicit dependencies. 
+        run: |
+          pip install uv
+          uv pip install --system --resolution=lowest .[all,test]
 
       # We nuke the local copy, as we've installed synapse into the virtualenv
       # (rather than use an editable install, which we no longer support). If we