Restrict allowed chars for Kubernetes Agent's name

What does this MR do?

The validation rules comes from the Kubernetes specification for the name field. See https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names

No Changelog necessary as there is no user-facing method to create or edit agents as yet

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.