fix: #3627197 Drop core calls removed or deprecated in Drupal 12

Issue: https://www.drupal.org/project/webdashboard/issues/3627197

• SystemInfo::getCounter(): RequirementSeverity::convertLegacyIntSeveritiesToEnums() is removed in Drupal 12. Requirements already come as enums; a legacy integer from a contrib module on Drupal 11 is mapped with RequirementSeverity::from().
• update_get_available() stays: it still exists on Drupal 12, and Drupal 11.4 has no service replacement for it.
• The branch was already red before this MR: Display Builder 1.0.0-beta8 added DisplayBuildableInterface::getTranslationLanguages(), so both buildables were fatal. They now return the default language only (dashboards are not translated in Display Builder).
• DashboardUiTest: Drupal 12 has no Olivero or Claro. The test now uses Stark for the front end and Claro (11) or Default Admin (12) for the back end.
• CI: opt in to the next-major and max-PHP jobs.

AI-Generated: Yes

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