REST endpoint xpack.ssl.certificates should be renamed

[clintongormley]

The xpack.ssl.certificates REST endpoint in the REST spec should be renamed to xpack.security.ssl_certificates, as ssl is not a namespace we have or need.

/cc @elastic/es-clients

[elasticmachine]

Pinging @elastic/es-security

[Mpdreamz]

The rename could potentially go in in 7.x where we make ssl.certificates deprecated as per #39063
and copy the spec to security.ssl_certificates.

A hard rename on 6.x from xpack.ssl.certificates to xpack.security.ssl_certificates on 6.x would be breaking.

Assuming you meant on 7.x since that's the one you linked but calling it out explicitly nonetheless 😄