Enforce strict teleport.yaml validation

[awly]

Strict validation was added in warning mode in
#5057 and released in 6.0.

For 7.0, we can drop the legacy custom validation logic, with the
assumption that all bad configs were migrated.

Fixes #5056
Fixes #5559

[webvictim]

IMHO we shouldn't merge this without adding support for teleport configure --check as described in #5559.

It's quite likely that many users have errors in their Teleport configs and have just never realised despite us showing a warning. At DEBUG level, the console warning scrolls past within a few seconds (despite the deliberate sleep) and if you view logs with journalctl -u teleport while configuring Teleport, all the logs tend to blend together. Merging this is likely to break configs which currently function despite the errors and increase support load. It may also lock people out of remote nodes until they fix their configs, requiring a backdoor for entry. With a teleport configure --check, people could validate their configs out of band without needing to restart Teleport.

[awly]

@webvictim done
@klizhentas PTAL

[webvictim]

Tested, works great. Thanks!