openssl: revert keylog_callback support for LibreSSL

- Revert to the legacy TLS 1.2 key logging code for LibreSSL.

- Document SSLKEYLOGFILE for LibreSSL is TLS 1.2 max.

Prior to this change if the user specified a filename in the
SSLKEYLOGFILE environment variable and was using LibreSSL 3.5.0+ then
an empty file would be created and no keys would be logged.

This is effectively a revert of e43474b4 which changed openssl.c to use
SSL_CTX_set_keylog_callback for LibreSSL 3.5.0+. Unfortunately LibreSSL
added that function only as a stub that doesn't actually do anything.

Reported-by: Gonçalo Carvalho

Fixes https://github.com/curl/curl/issues/13672
Closes https://github.com/curl/curl/pull/13682

docs/cmdline-opts/_ENVIRONMENT.md

@@ -101,8 +101,8 @@ if Schannel is used as the TLS backend.
 If you set this environment variable to a filename, curl stores TLS secrets
 from its connections in that file when invoked to enable you to analyze the
 TLS traffic in real time using network analyzing tools such as Wireshark. This
-works with the following TLS backends: OpenSSL, libressl, BoringSSL, GnuTLS
-and wolfSSL.
+works with the following TLS backends: OpenSSL, LibreSSL (TLS 1.2 max),
+BoringSSL, GnuTLS and wolfSSL.
 
 ## `USERPROFILE` <dir>
 On Windows, this variable is used when trying to find the home directory. If