Remove windows-dns-proxy feature flag

[vvoland]

Description

• related to: Windows DNS resolver forwarding #47584

The windows-dns-proxy feature flag was added in the v26.1 release as a way to opt-in into the new behavior.
Starting from v27.0, we intend to make the new behavior a default and turn the flag into an opt-out.

This flag should be removed once we verify that the new behavior doesn't break users workflow.