tls: make default TLS version be minimum 1.2

tls: make default TLS version be minimum 1.2

This still allows users to explictily ask for 1.0 or 1.1 as the minimum
version. If the TLS library allows it.

Starting with this change, the CURL_SSLVERSION_DEFAULT value is no
longer used as minimum version when the TLS backend are called.

This also makes curl set the minimum version to 1.2 independently of
libcurl for the rare case where a newer curl tool would use an older
libcurl.

URL: https://curl.se/mail/lib-2025-07/0007.html
Assisted-by: Stefan Eissing
Closes #17894