Temporarily remove Ultimate Cron until it has a stable release, to resolve on Stable for Drupal CMS

Problem/Motivation

drupal/ultimate_cron has no stable release, so a project on Composer's default minimum-stability: stable cannot install a recipe that requires it:

drupal/ultimate_cron ~2 -> found drupal/ultimate_cron[2.0.0-alpha1, ...] but it does not match your minimum-stability.

That is what blocks the Varbase site templates on Drupal CMS, where a site template has to resolve on stable.

This recipe does not enable the module: ultimate_cron is not in its install: list. It does ship one job definition, config/ultimate_cron.job.search_api_cron.yml, which only imported because another recipe had enabled the module first.

We asked the maintainers for a stable 2.x release in #2660208. The same removal in Varbase Performance Base, where the module really is installed, is #3621493.

This removal is temporary. Ultimate Cron goes back if this recipe needs it once it is stable.

Steps to reproduce

1. On a project with the default minimum-stability: stable, run composer require drupal/varbase_search_base:~1.0.0.
2. Resolution fails on drupal/ultimate_cron.
3. Compare composer.json with recipe.yml: the package is required but never installed by this recipe.

Proposed resolution

• Remove drupal/ultimate_cron from composer.json.
• Remove config/ultimate_cron.job.search_api_cron.yml.
• Update the module list in README.md.

No change to recipe.yml. Search indexing keeps running on whatever cron the site uses.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Release

User interface changes

• None. This recipe never installed the module.

API changes

• drupal/ultimate_cron is no longer required by this recipe.

Data model changes

• One ultimate_cron.job config object is no longer shipped.

Release notes snippet

• Temporarily stop requiring Ultimate Cron, which has no stable release and was never installed by this recipe, so it resolves on a project with the default stable stability. See #2660208.