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))

.ci/scripts/prepare_old_deps.sh

@@ -1,39 +0,0 @@
-#!/usr/bin/env bash
-# this script is run by GitHub Actions in a plain `jammy` container; it
-# - installs the minimal system requirements, and poetry;
-# - patches the project definition file to refer to old versions only;
-# - creates a venv with these old versions using poetry; and finally
-# - invokes `trial` to run the tests with old deps.
-
-set -ex
-
-# Prevent virtualenv from auto-updating pip to an incompatible version
-export VIRTUALENV_NO_DOWNLOAD=1
-
-# TODO: in the future, we could use an implementation of
-#   https://github.com/python-poetry/poetry/issues/3527
-#   https://github.com/pypa/pip/issues/8085
-# to select the lowest possible versions, rather than resorting to this sed script.
-
-# Patch the project definitions in-place:
-# - `-E` use extended regex syntax.
-# - Don't modify the line that defines required Python versions.
-# - Replace all lower and tilde bounds with exact bounds.
-# - Replace all caret bounds with exact bounds.
-# - Delete all lines referring to psycopg2 - so no testing of postgres support.
-# - Use pyopenssl 17.0, which is the oldest version that works with
-#   a `cryptography` compiled against OpenSSL 1.1.
-# - Omit systemd: we're not logging to journal here.
-
-sed -i -E '
-  /^\s*requires-python\s*=/b
-  s/[~>]=/==/g
-  s/\^/==/g
-  /psycopg2/d
-  s/pyOpenSSL\s*==\s*16\.0\.0"/pyOpenSSL==17.0.0"/
-  /systemd/d
-' pyproject.toml
-
-echo "::group::Patched pyproject.toml"
-cat pyproject.toml
-echo "::endgroup::"