fix: Remove Invalid PodSecurityContext Values

What does this MR do?

#6398 (closed) points out that in !4683 (merged) some invalid PodSecurityContext fields were added. Per the PodSecurityContext Object Definition, privileged and allowPrivilegeEscalation are not valid at the pod level and should only be provided for containers.

I've attempted to make this as small of a change as possible - updating the podSecurityContext helpers to remove privileged and allowPrivilegeEscalation, and updating spec/configuration/gitaly_spec.rb to verify podSecurityContext after these changes. I don't believe this is a breaking change, as this is essentially reverting functionality that wouldn't have worked previously.

I ensured that helm template with no values provided and helm template with the values provided in the above issue both succeed and do not return the unwanted fields in the podSecurityContext.

Related issues

Closes #6398 (closed)

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