Unnecessary auto-upgrade-enroll message emitted at WARN level

[webvictim]

Expected behavior

All Teleport warnings should provide real errors with actionable feedback for users.

Current behavior

With a stock config from teleport configure, ~45 seconds after startup and every ~8 minutes after that, Teleport 13.2.3 and later emit a message at WARN level:

Aug 23 10:19:33 ubuntu teleport[17262]: 2023-08-23T10:19:33-03:00 WARN [AUTH]      Failed to delete auto-upgrade-enroll alert: cluster alert "auto-upgrade-enroll" not found auth/auth.go:1101
Aug 23 10:27:58 ubuntu teleport[17262]: 2023-08-23T10:27:58-03:00 WARN [AUTH]      Failed to delete auto-upgrade-enroll alert: cluster alert "auto-upgrade-enroll" not found auth/auth.go:1101

This message is not actionable and confuses users, thinking there is a problem with their cluster when it's just running normally. There has been an uptick in logs in the Community Slack mentioning this message as a problem with the cluster in the last week.

We should suppress or otherwise remove this message from log output.

@fspmarshall git blame has pointed me at #28951 and its v13 backport #29206 as being the likely culprit.

[fspmarshall]

Fixed in #30849