Use HTTP readiness and liveness probes in KAS service

What does this MR do?

This is the first iteration to bring proper readiness and liveness probes to the KAS chart. There maybe future plans to add probes via gRPC.

The probe endpoints have been implemented a while ago with gitlab-org/cluster-integration/gitlab-agent!149 (merged), but haven't actually been used in this Chart.

This change set also deprecates the gitlab.kas.metrics.port config value in favor of gitlab.kas.observability.port.

Related issues

Issue Reference: gitlab-org/cluster-integration/gitlab-agent#18 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.