## Problem
https://github.com/neondatabase/neon/pull/10841 broke CI on PRs that
aren't based on main or a release branch but want to merge into another
PR.
## Summary of changes
Replace `run-kind=pr-main` with `run-kind=pr`, so that all PRs that
aren't release PRs are treated equally.
## Problem
Release CI is slow, because we're doing unnecessary work, for example
building compute images on storage releases and vice versa.
## Summary of changes
- Extract tag generation into reusable workflow and extend it with
fetching of previous component releases
- Don't build neon images on compute releases and don't build compute
images on proxy and storage releases
- Reuse images from previous releases for tests on branches where we
don't build those images
## Open questions
- We differentiate between `TAG` and `COMPUTE_TAG` in a few places, but
we don't differentiate between storage and proxy releases. Since they
use the same image, this will continue to work, but I'm not sure this is
what we want.
