Add a sync-release action to automate updating to the latest release

[merlijn-sebrechts]

I created the stable branch environment and added the snapcrafters-bot commit token to it. You will need to generate and add the other tokens yourself as described here: https://forum.snapcraft.io/t/automatic-snap-refreshes-using-github-actions-snapcrafters-guide/33444

You also need two more changes to use the new secrets from the environments:

• https://github.com/snapcrafters/signal-desktop/commit/0455effe9cbcb7d584865cf1c902a486ebf4d483
• https://github.com/snapcrafters/signal-desktop/commit/4f200ef157b15482f00e85071a99b3b4f179cce7

edit: just saw this is only the sync-release so you only need one change and you don't need to generate any new secrets

Let me know if anything's unclear!

[jnsgruk]

Closing in favour of #81