Handle remaining TLS 1.2 names

[davidben]

Implementations of TLS 1.2 will remain for some time. The document renames "master secret" to "main secret", but TLS 1.2 has some naming variations that aren't in TLS 1.3. Perhaps a section in the document somewhere (maybe under Backward Compatibility since that talks about older versions) that describes some additional deltas:

• "master secret" => "main secret", to align this this document
• "premaster secret" => "preliminary secret" ("pre-main secret" sounds weird to me)
• "extended master secret" => "extended main secret" (probably move rfc7627 from informative to normative and then also add to the updates line)
• Also cover the variations like "EncryptedPreMasterSecret" and "master_secret", but not the KDF labels since those can't change.

And then IANA considerations to rename extended_master_secret to extended_main_secret in the registries.