Remove support for TLS 1.0 and 1.1
Following https://webkit.org/blog/8462/deprecation-of-legacy-tls-1-0-and-1-1-versions/
We should drop support for TLS 1.0 and 1.1 after branching glib-networking 2.62. i.e. 2.64 (targeted for March 2020 release) would be the first version without support for these legacy protocols.