Use "critical" deprecation level for elasticsearch.username: elastic configuration

[jportner]

In #51101, we outlined our intent to prevent Kibana from authenticating to Elasticsearch using the elastic superuser in production. (We already prevent this in dev mode)

At first we'd hoped to make that breaking change in the 8.0 release, but we decided to defer it along with several other planned breaking changes.

Now, elastic/elasticsearch#81400 will change the superuser role to remove write access to system indices. That will implicitly prevent Kibana from using the elastic superuser to authenticate to Elasticsearch, since Kibana needs to be able to write to system indices.

In #115241, we made a change to treat this as a "warning" deprecation level in the Upgrade Assistant. We need to instead treat it as a "critical" deprecation level in the 7.17 release, because this configuration will no longer work starting in 8.0.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)