SSH key rotation

1. Generate a new key in /root/keyholder. Suggested naming convention is l10n-bot-YYYY. Protect the key with a passphrase and it it to the vault
2. Make a commit like https://gerrit.wikimedia.org/r/c/translatewiki/+/881425
3. Manually restart and arm keyholder-agent and keyholder-proxy
4. Confirm that it works, the order of restarting can be fiddly
5. Replace ssh key in all forges
6. Remove the old key