Update Mullvad how-to to reflect unencrypted public DNS being shutdown

[Spencer]

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• I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• I have searched the existing issues and I am convinced that mine is new.

Describe the issue

The Mullvad how-to instructs users to use 193.138.218.74. This was Mullvad's unencrypted public DNS, and has since been shutdown: https://mullvad.net/en/blog/shutting-down-our-unencrypted-public-dns-service

Suggestions

Update the docs so that this IP is no longer reference and whatever replaces it (DNS over TLS?) is used.

Version affected
docs n/a