Bump CORE_STABLE to 10.3.1

Migrated issue

Reported by: jonathan1055

Related to !235 (merged)

Problem/Motivation

Drupal core 10.3.1 has been released. Our 'check versions' job shows the new improved output

check versions

https://git.drupalcode.org/issue/gitlab_templates-3458411/-/jobs/2040651