gbp-notifications 2.0.0

This release has a backwards-incompatible change, described below, hence
the major version bump.

Notable changes:

Python 3.11 is no longer supported as Gentoo Build Publisher itself
has dropped support.
"Events" in gbp-notifications now are the same names as Gentoo
Build Publisher's signal dispatcher signal names. For the most part
this means that the previous "build_pulled" event is now called
"postpull" and "build_published" is now called "published". This
change allows us to arbitrarily support any GBP signal as well as
signals from other GBP plugins. This is a breaking change.
The webhook method now uses custom User-Agent header when
communicating with the web server.

Full Changelog: v1.2.1...v2.0.0