This should ensure GHA runs synapse against the same-named sytest branch

use PULL_REQUEST_NUMBER instead of BUILDKITE_PULL_REQUEST
remove the other user of BUILDKITE_PULL_REQUEST, namely merge_base_branch.sh

Updates CI and the helper script t ensures all tests are run (in parallel).

A few things here:

* Build the debs for single distro for each PR, so that we can see if it breaks. Do the same for develop. Building all the debs ties up the GHA workers for ages.
* Stop building the debs for release branches. Again, it takes ages, and I don't think anyone is actually going to stop and look at them. We'll know they are working when we make an RC.
* Change the configs so that if we manually cancel a workflow, it actually does something.

This implements similar behavior to sytest where a matching branch is used,
if one exists. This is useful when needing to modify both application code
and tests at the same time. The following rules are used to find a matching
complement branch:

1. Search for the branch name of the pull request. (E.g. feature/foo.)
2. Search for the base branch of the pull request. (E.g. develop or release-vX.Y.)
3. Search for the reference branch of the commit. (E.g. master or release-vX.Y.)
4. Fallback to 'master', the default complement branch name.

This PR updates the build tags that we perform Complement runs with to match our [buildkite pipeline](https://github.com/matrix-org/pipelines/blob/618b3e90bcae8efd1a71502ae95b7913e6e24665/synapse/pipeline.yml#L570), as well as adding `msc2403` (as it will be required once #9359 is merged). Build tags are what we use to determine which tests to run in Complement (really it determines which test files are compiled into the final binary).

I haven't put in a comment about updating the buildkite side here, as we've decided to migrate fully to GitHub Actions anyhow.

... using the script from matrix-org/sytest#1052

* Fix GitHub Actions lint for newsfragments

Signed-off-by: Dan Callahan <danc@element.io>

... and test it.

Signed-off-by: Dan Callahan <danc@element.io>