This commit refactors the proto definitions of the
reservation notifications to use a more generic
mechanism that can be used for other types of
notifications as well.
This is needed to update protobuf. Version 1.33 breaks in Go 1.16 with
the following error: "//go:build comment without // +build comment".
Distribution was updated from buster (10) to bookworm (12).
protoc was updated from v3.6.1 to v3.21.12.
