Clarify file permissions needed for gitserver ssh key

[caugustus-sourcegraph]

https://docs.sourcegraph.com/admin/install/kubernetes/configure#configure-repository-cloning-via-ssh says that the SSH key secret should be mounted with 0644 permissions. In the kustomize base files and overlays, 384 is specified (translates to 0600 in octal).

Determine whether 0600 is sufficient permission, or if 0644 is needed, and update the references to be consistent.

[github-actions]

Heads up @dan-mckean @caugustus-sourcegraph @kevinwojo - the "team/delivery" label was applied to this issue.

[virginiaulrich]

Good first issue for a new hire to take on; so leave in backlog with that in mind