fix: only merge older RCs with open release PRs (MCRM-80)

[sleepytanya]

The merge-previous-releases workflow was merging all older release/* branches, including obsolete ones already merged to stable. Now it queries GitHub for open/draft PRs targeting stable and only merges those active RCs.

https://consensyssoftware.atlassian.net/browse/MCRM-80

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Note

Medium Risk
Changes release-branch merge automation in CI; wrong PR filtering or gh/token failures could skip needed merges or block new release setup.

Overview
The merge-previous-releases script no longer merges every older release/* remote branch into a newly cut release branch. It now uses gh pr list to collect open PRs whose base is stable and whose head matches release/X.Y.Z, then merges only those active RCs that are still older than the new branch (same version sort and conflict handling as before).

If the GitHub API query fails (e.g. token permissions), the script exits with an error instead of silently skipping merges. Docs and log messages were updated accordingly, and CHANGELOG records the fix under Unreleased.

^{Reviewed by Cursor Bugbot for commit f8da6c3. Bugbot is set up for automated code reviews on this repo. Configure here.}

[Gudahtt]

LGTM!

I discovered this isn't actually breaking

[Gudahtt]

Suggested change

	### Changed
	- **BREAKING:** `merge-previous-releases` now requires `GITHUB_TOKEN` to query open release PRs via GitHub API ([#282](https://github.com/MetaMask/github-tools/pull/282))

Sorry for the mixup, I just discovered this isn't actually a breaking change. The new requirement of the GITHUB_TOKEN for this script doesn't impact the action, because it already required GITHUB_TOKEN to checkout this repo itself

[sleepytanya]

Removed! f8da6c3