task: #3627196 Suggest the xAI and Mistral providers instead of requiring them

Issue: https://www.drupal.org/project/webai/issues/3627196

• drupal/ai_provider_xai moves from require to suggest, and out of the default recipe's install list. Its grok-php/client needs Guzzle 7; Drupal 12 uses Guzzle 8, so Composer can't install Web AI on Drupal 12 while xAI is required.
• drupal/ai_provider_mistral too: its partitech/php-mistral needs Symfony 6 or 7, and Drupal 12 uses Symfony 8 (found by the next-major job).
• README: five installed providers, plus how to add xAI on Drupal 11. A site that already has xAI enabled should require it in its own composer.json before updating.
• CI: opt in to the next-major and max-PHP jobs.

AI-Generated: Yes

Checkpoints

• File an issue about this project
• Addition/Change/Update/Fix to this project
• Testing to ensure no regression
• Automated unit/functional testing coverage
• Developer Documentation support on feature change/addition
• User Guide Documentation support on feature change/addition
• UX/UI designer responsibilities
• Accessibility and Readability
• Reviewed by a human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Update Release Notes
• Release