Readd the __proxy_keepalive scheduled job
Readd the __proxy_keepalive scheduled job

Although added by @cro in #38829 - see
https://github.com/saltstack/salt/pull/38829/files#diff-b5732eecf0273ccf6a96e1c720198dc6
for very obscure reasons, these changes somehow disappeared.
Readding them, hopefully they'll resist longer this time.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt