Allow milestone string to include "<"

What does this MR do and why?

As part of #478242 we would like to be able to set a milestone of e.g. <13.9 to indicate that an event was introduced before 13.9. We already allow this for metric definitions and after this change we have the same validation rule for milestones in events and metric definitions.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. On master: Modify config/events/1643968255_projectsnew_select_deployment_target.yml to have milestone "<14.8"
2. Run the spec spec/lib/gitlab/tracking/event_definition_validator_validate_all_spec.rb and verify that it fails
3. Try the same on this branch and see the spec pass ✅

Related to #478242