Backport Bugzilla 1915093: Don't prefetch HTTPS RR if proxyDNS is enabled

Backport Patchset

Book-keeping

Gitlab Issue(s)

• #387 (closed)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1910593

Merge Request(s)

• !150 (merged)

Target Channels

• Stable
• Legacy

Notes

This is currently justin Mullvad Browser Alpha, per discussion on the issues it looks like we want to backport to Stable, but also to Tor Browser presuming the patch has been verified.

/cc @pierov