server: minimum TLS version configuration, default to 1.2 (#3517)

* Support for minimum TLS version

OPA server now supports min TLS version, TLS versions supported are 1.0, 1.1, 1.2, 1.3.

Since TLS 1.0 and 1.1 are deprecated, default min TLS version for OPA is TLS 1.2 but
if someone wants to restrict OPA to use a specific minimum TLS version, they can
specify it using cmd parameter `--min-tls-version`.

Fixes #3226.

Signed-off-by: Amruta Kale <amruta.kale@styra.com>