Doesn't know how to merge up new repositories

[GuySartorelli]

Failed merge-up in https://github.com/silverstripe/markdown-php-codesniffer/actions/runs/6973288579/job/18977009246 is due to faulty or missing logic in this action:

PHP Fatal error: Uncaught Exception: Could not work out what the default CMS major version this module uses in /home/runner/work/_actions/silverstripe/gha-merge-up/v1/funcs.php:87
Stack trace:
#0 /home/runner/work/_actions/silverstripe/gha-merge-up/v1/branches.php(9): branches()
#1 {main}
thrown in /home/runner/work/_actions/silverstripe/gha-merge-up/v1/funcs.php on line 87

Acceptance criteria

• The merge-up action in silverstripe/markdown-php-codesniffer works without errors
• All new repositories will work without needing a change each time one is added
  OR
• The instructions for setting up a new repository include whatever steps are necessary to get merge-ups working.

PRs

• DEP Change minimum PHP version to 8.1 markdown-php-codesniffer#4
• DOC Document CMS major version detection #29

[GuySartorelli]

@emteknetnz The fragility of merge-ups requiring a specific minimum PHP version should be documented, either in the merge-ups readme or the "adding a new repo" page.

[emteknetnz]

Done

[GuySartorelli]

Merged