CI: use plain synapse docker images for testing

[bnjbvr]

Both integration testing + code coverage use an image labelled synapse-service, which is likely outdated (points to the matrix-org org) and hard to find. We should likely use the regular synapse Docker image for testing, if we could.

[bnjbvr]

If we use the synapse docker image, and it's easy to enable experimental features (which is how I started looking into this), we should reenable the test disabled with the last commit of #3339.

[kegsay]

Unless things have changed with Synapse images, it will be much less ergonomic to use the official images which is why synapse-service exists. It's less ergonomic because you can't just configure everything with env vars (like you can with say the postgres docker image which is trivial to use as a GHA service) and will need to faff about with the YAML config file and generating server keys.

Neither the official image nor the complement image make it easy to enable experimental MSCs sadly. The enabled set of MSCs in the Complement Synapse image is hard-coded at https://github.com/element-hq/synapse/blob/develop/docker/complement/conf/workers-shared-extra.yaml.j2#L106

I suspect the "best" solution here would be to pester me / someone from backend to improve the Synapse Complement image to:

• not crash if you don't give it a CA certs directory (this is why I made synapse-service in the first place)
• make it possible to enable MSCs via env vars.

And then you can just use the Synapse Complement image as a drop-in replacement. The env vars in that image exactly match the ones in synapse-service.