Adds readiness probe for gitlab-shell

What does this MR do?

• Configures the readiness probe for the gitlab-shell to utilize a tcp check for port 2222
  • This is hardcoded as it is hardcoded in the CNG configuration and is therefore not an item that can be changed
  • We do not rely on the healthcheck script as this will accidentally match the pgrep which is not precise when the sshd is not running but clients may still be connected
• Set the default values of the readiness probes to be slightly shorter than the liveness probes to allow for a Pod to go into an unhealthy state, removing it from service, prior to being restarted
  • This should hopefully allow for existing connections to complete vs becoming interrupted during deploys or incidents with a given Pod
• Adds an example configuration explaining the use case of potentially requiring more fine tuning of this configuration

Addresses: #2357 (closed)

Related issues

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

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