Fall back to auto-merge + branch update when direct merge fails

[ajpallares]

Summary

• When the direct merge of a release PR fails (e.g. because the branch is not up-to-date with main), and merge queue is not in use, the merge-release-pr job now falls back to:
  1. Enabling auto-merge on the PR (with the same merge method)
  2. Updating the PR branch by merging the base branch into it via the GitHub API

This handles repos like purchases-flutter that have the "Require branches to be up to date before merging" branch protection rule. See this pipeline for an example of the failures this fixes.

Depends on: fastlane-plugin-revenuecat_internal#125 (new update_pr_branch action).

Test plan

• Verify the orb packs and lints successfully
• Test on a repo with "require up-to-date" branch protection by triggering a release where main has diverged
• Verify merge queue path still fails normally on enqueue errors

[RCGitBot]

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
https://circleci.com/developer/orbs/orb/revenuecat/sdks-common-config?version=dev:3cafe27d3a471e01ac69bed80d2378c050c2fc62

[RCGitBot]

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
https://circleci.com/developer/orbs/orb/revenuecat/sdks-common-config?version=dev:3cbc6fbe36928e01cb20dbf22869dbc9a92a429c

[rickvdl]

Smart solution :) LGTM!