feat: consolidate TLS options with rustls-platform-verifier

[seanmonstar]

This includes two logical change groups, done together because the intermediate step is not really useful.

1. This makes rustls-platform-verifier the default verifier, and removes the ability to use rustls with webpki or native roots directly.
2. This soft-deprecates many of the TLS-related builder methods, providing better names that are clearer and autocomplete better.

This greatly simplifies the configuration matrix of what could be enabled, but also means the removal of some parts that some people may have been using. The consolidated API should still allow users to do these things.

For example, instead of adding webpki-roots with a feature flag, you should use the webpki-roots crate yourself, and pass them into tls_certs_only(certs).

Closes #2885

[ofek]

Thanks! Heads up that the final sentence of the first step in the PR description appears to have been cut off.