Investigate how MBS relies on fedmsg and its implications on us when fedmsg disappears

It seems MBS still relies on fedmsg which we expect to disappear in the near future.

We still need to run MBS until the c9s EOL.

Investigate and report back on:

• how MBS uses fedmsg
• when can we expect fedmsg to disappear
• and estimage how much work would it be to migrate it to Fedora Messaging if needed

(This is just investigation work so we know what's awaiting us, and have time to plan / escalate risks.)