Add GitLab timestamps to gitlab-shell/ssh.log

For debugging SSH problems, and investigating performance issues with it, having timestamps by GitLab included in the gitlab-shell/ssh.log file would be helpful.

Currently, it is not clearly possible to correlate events with those in gitlab-shell/gitlab-shell.log, or with customer-provided ssh -vvv traces.

Log event examples:

• internal customer in GitLab.org / gitlab-shell case: ZD 277460
• public examples: gitlab-org/gitlab-shell#157 (closed) & these in gitlab-org/charts

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Also (might be a separate issue): gitlab-shell/ssh.log seems undocumented generally.