ci: add automatic backport workflow (#24025)

Adds a GitHub Actions workflow that automatically cherry-picks merged
PRs to the last 3 release branches when the `backport` label is applied.

## How it works

1. Add the `backport` label to any PR targeting `main` (before or after
merge).
2. On merge (or on label if already merged), the workflow discovers the
latest 3 `release/*` branches by semver.
3. For each branch, it cherry-picks the merge commit (`-x -m1`) and
opens a PR.

Created backport PRs follow existing repo conventions:
- **Branch:** `backport/<pr>-to-<version>`
- **Title:** `<original PR title> (#<pr>)` — e.g. `fix(site): correct
button alignment (#12345)`
- **Body:** links back to the original PR and merge commit

If cherry-pick has conflicts, the PR is still opened with instructions
for manual resolution — no conflict markers are committed.

Also:
- Removes `scripts/backport-pr.sh` (replaced by this workflow)
- Removes `.github/cherry-pick-bot.yml` (old bot config)
- Adds a section to the contributing docs explaining how to use the
backport label

> [!NOTE]
> Generated with [Coder Agents](https://coder.com/agents)

docs/about/contributing/CONTRIBUTING.md

@@ -291,18 +291,22 @@ specification, however, it's still possible to merge PRs on GitHub with a badly
 formatted title. Take care when merging single-commit PRs as GitHub may prefer
 to use the original commit title instead of the PR title.
 
-### Cherry-picking to the latest release branch
+### Backporting fixes to release branches
 
-When a merged PR on `main` should also ship in the current release,
-add the **`cherry-pick`** label to the PR (before or after merge).
+When a merged PR on `main` should also ship in older releases, add the
+`backport` label to the PR. The
+[backport workflow](https://github.com/coder/coder/blob/main/.github/workflows/backport.yaml)
+will automatically detect the latest three `release/*` branches,
+cherry-pick the merge commit onto each one, and open PRs for
+review.
 
-The automation detects the latest `release/*` branch, cherry-picks the
-merge commit, and opens a PR for review. The PR reuses the original
-title (e.g. `fix(site): correct button alignment (#12345)`) so the
-change is meaningful in release notes.
+The label can be added before or after the PR is merged. Each backport
+PR reuses the original title (e.g.
+`fix(site): correct button alignment (#12345)`) so the change is
+meaningful in release notes.
 
-If the cherry-pick encounters conflicts, the PR is still created with
-instructions for manual resolution.
+If the cherry-pick encounters conflicts, the backport PR is still created
+with instructions for manual resolution — no conflict markers are committed.
 
 ### Breaking changes