Deprecate FF_RESOLVE_FULL_TLS_CHAIN, to be removed in 20.0

What does this MR do?

Deprecate FF_RESOLVE_FULL_TLS_CHAIN, to be removed in 20.0

The flag has defaulted to false since 16.4 (!4292 (merged)). It is the only entry point to the ca_chain resolver, which in turn is the only consumer of the unmaintained PKCS7 dependency (see !6016 (merged), #39205). Follows the "deprecating full chain resolving entirely" suggestion in !6016 (merged)#note_2963136182, confirmed with the flag's original author on #39694 (closed).

Also scope the pre-16.4 workaround guidance in docs/install/osx.md and docs/configuration/tls-self-signed.md to 15.5.x to 16.3, so the advice is no longer a no-op on all supported versions.

Why was this MR needed?

To mark this FF as deprecated.

What's the best way to test this MR?

What are the relevant issue numbers?

Deprecate FF_RESOLVE_FULL_TLS_CHAIN (#39694 - closed)