DEPRECATE.md: TLS libraries without 1.3 support

curl drops support for TLS libraries without TLS 1.3 capability after
May 2025.

It requires that a curl build using the library should be able to
negotiate and use TLS 1.3, or else it is not good enough. We support a
vast amount of other TLS libraries that are likely to satisfy users
better.

Closes #13544

docs/DEPRECATE.md

@@ -12,6 +12,17 @@ email the
 as soon as possible and explain to us why this is a problem for you and
 how your use case cannot be satisfied properly using a workaround.
 
+## TLS libraries without 1.3 support
+
+curl drops support for TLS libraries without TLS 1.3 capability after May
+2025.
+
+It requires that a curl build using the library should be able to negotiate
+and use TLS 1.3, or else it is not good enough.
+
+As of May 2024, the libraries that need to get fixed to remain supported after
+May 2025 are: BearSSL and Secure Transport.
+
 ## space-separated `NOPROXY` patterns
 
 When specifying patterns/domain names for curl that should *not* go through a