ci: skip reverted commits in release notes, publish tagged snaps to stable (#39657) (#39678)

Backport #39657 by @bircni

Release notes: revert commits and the commits they revert are now left
out of the git-cliff changelog. A step before git-cliff finds commits
whose subject starts with "revert", reads the PR numbers on their revert
lines, and writes both SHAs to `.cliffignore`.

Snap: since the move from Launchpad to GitHub Actions, every build was
uploaded to `latest/edge`, including the stable-grade build that
`part-gitea-pull.sh` makes for an unreleased tag. Launchpad used to
release those builds to candidate and stable automatically. Without
that, v28.0.0 had to be promoted by hand and v28.1.0 stayed in edge.
Stable-grade snaps now go to `latest/stable` and `latest/candidate`;
main builds still go to edge. Candidate has to be updated too because
the pull script compares against it.

Signed-off-by: bircni <bircni@icloud.com>
Co-authored-by: bircni <bircni@icloud.com>

tools/generate-cliffignore.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+set -euo pipefail
+
+# Prints the revert commits in the given range and the commits they revert, for git-cliff's .cliffignore
+range="${1:?usage: $0 <revision-range>}"
+commits=$(git log --format='%H %s' "$range")
+
+shopt -s nocasematch
+while read -r sha subject; do
+  [[ "$subject" == revert* ]] || continue
+  echo "$sha"
+  for pr in $(git show -s --format=%B "$sha" | grep -i revert | grep -oE '(#|/pull/)[0-9]+' | grep -oE '[0-9]+' || true); do
+    grep -F "(#$pr)" <<< "$commits" | cut -d' ' -f1 || true
+  done
+done <<< "$commits"