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_CERTINFO.md

@@ -14,7 +14,6 @@ TLS-backend:
   - OpenSSL
   - GnuTLS
   - Schannel
-  - Secure Transport
   - rustls
 Added-in: 7.19.1
 ---
@@ -97,8 +96,8 @@ See also the *certinfo.c* example.
 
 # HISTORY
 
-GnuTLS support added in 7.42.0. Schannel support added in 7.50.0. Secure
-Transport support added in 7.79.0. mbedTLS support added in 8.9.0.
+GnuTLS support added in 7.42.0. Schannel support added in 7.50.0. mbedTLS
+support added in 8.9.0.
 
 # %AVAILABILITY%