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. 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 blocks every Varbase site template on Drupal CMS, where a site template has to resolve on stable. All four templates apply this recipe directly.

We asked the maintainers for a stable 2.x release in #2660208.

This removal is temporary. Ultimate Cron goes back when it has a stable release.

This is a real change, not a dependency cleanup: the module is in the install: list and this recipe's whole config/ directory is its sixteen job definitions. An existing site keeps the module and its jobs, and vardot/varbase keeps requiring it in #3621480.

Steps to reproduce

1. On a project with the default minimum-stability: stable, run composer require drupal/varbase_performance_base:~1.0.0.
2. Resolution fails on drupal/ultimate_cron.

Proposed resolution

• Remove drupal/ultimate_cron from composer.json.
• Remove ultimate_cron from the install: list in recipe.yml.
• Remove the sixteen config/ultimate_cron.job.*.yml files. A recipe imports everything in config/, so leaving them behind fails the apply once the module is gone.
• Update the module list in README.md.

A new site falls back to core's Automated Cron, which this recipe already configures with automated_cron.settings.interval: 10800. What it loses is per-job scheduling, per-job logging and the cron overview screen.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ✅ Reviewed by a human
• ✅ Code review by maintainers
• ✅ Release varbase_performance_base-1.0.1

User interface changes

• A new site applying this recipe has no Ultimate Cron jobs screen. Cron runs through core's Automated Cron.

API changes

• This recipe no longer requires or installs drupal/ultimate_cron, and no longer ships its job configuration.

Data model changes

• Sixteen ultimate_cron.job.* config objects are no longer created on a new site.

Release notes snippet

• Temporarily stop requiring and installing Ultimate Cron, which has no stable release, so this recipe resolves on a project with the default stable stability. Cron runs through core's Automated Cron in the meantime. An existing Varbase site keeps the module through the profile.