TLS: remove support for Secure Transport and BearSSL

These libraries do not support TLS 1.3 and have been marked for removal
for over a year. We want to help users select a TLS dependency that is
future-proof and reliable, and not supporting TLS 1.3 in 2025 does not
infer confidence. Users who build libcurl are likely to be served better
and get something more future-proof with a TLS library that supports
1.3.

Closes #16677

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.md

@@ -11,12 +11,10 @@ See-also:
 Protocol:
   - TLS
 TLS-backend:
-  - BearSSL
   - GnuTLS
   - mbedTLS
   - OpenSSL
   - Schannel
-  - Secure Transport
   - wolfSSL
 Added-in: 7.48.0
 ---
@@ -87,10 +85,6 @@ as well:
 
 **CtxtHandle ***
 
-## Secure Transport
-
-**SSLContext ***
-
 ## wolfSSL
 
 **SSL ***