Rename PyVAST Threat Bus to VAST Threat Bus

[dominiklohmann]

📔 Description

We planned to do this for a while now, so with the recent renaming of zmq-app to zmq it makes sense to do this breaking change now as well.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/threatbus, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Verify that I changed all instances of PyVAST Threat Bus.

[lava]

We might be missing some places, but there's still a whole month left to discover and fix them. So let's go!