tls: remove checks for DEFAULT

tls: remove checks for DEFAULT

Since 9d8998c99408e1adf, the setopt code changes input DEFAULT to an
actual more specific TLS version (1.2) for the backends to use and check
for.

This means that the default value (0L) cannot and should not actually be
used when the TLS backends run. This change adds asserts to verify that
and removes code that accepts the DEFAULT value as a valid version with
the TLS version functions' logic.

Applications can still set a specific lower version if they want (1, 1.0
or 1.1).

Closes #20453