Update Ruby version from 3.3.11 to 3.3.12

Updates RUBY_VERSION and NEXT_RUBY_VERSION from 3.3.11 to 3.3.12 to match the omnibus-gitlab bump, and keeps the Dockerfile RUBY_VERSION ARG defaults in sync.

Refs:

• https://gitlab.com/gitlab-org/omnibus-gitlab/-/work_items/9982
• gitlab-org/omnibus-gitlab!9634 (merged)

Please make sure this follows the Ruby upgrade guidelines.