Improve warning when using elasticsearch.ssl.certificate without elasticsearch.ssl.key and vice versa

[jportner]

Starting in 8.0, we should prevent Kibana from starting if elasticsearch.ssl.certificate without elasticsearch.ssl.key and vice versa. This configuration will not enable TLS client authentication to Elasticsearch, and is unsupported.

Starting in 7.6, we're warning the user via deprecation logs (see #54392), so we are safe to enforce this in 8.0 after properly documenting it as a breaking change.

[elasticmachine]

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

[jportner]

We originally intended to throw an error starting in 8.0, but in the interests of making upgrades easier, we are opting not to make that change.
The existing warning doesn't mention that this will be removed in 8.0, so we can safely close this issue.