luci-app-https-dns-proxy: Mullvad URL update

[Hekkfern]

Description:
According to the notes section in Mullvad DNS documentation, the "doh" URL for their DNS server is deprecated. This PR updates "luci-app-https-dns-proxy" to use the new, correct URLs before they completely remove them.
Also:

• added all the DNS variants they provide (read here).
• change bootstrap dns to use Mullvad's own IPv4 and IPv6

Please, @stangri, could you help me reviewing the changes?

• This PR is not from my main or master branch 💩, but a separate branch ✅
• Each commit has a valid ✒️ Signed-off-by: <[email protected]> row (via git commit --signoff)
• Each commit and PR title has a valid 📝 <package name>: title first line subject for packages
• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: (architecture, openwrt version, browser) ✅
• ( Preferred ) Mention: @ the original code author for feedback
• ( Preferred ) Screenshot or mp4 of changes:
• ( Optional ) Closes: e.g. openwrt/luci#issue-number
• ( Optional ) Depends on: e.g. openwrt/packages#pr-number in sister repo
• Description: (describe the changes proposed in this PR)

[stangri]

Thank you for your contribution! Could you please update:

1. PKG_VERSION:=2023.12.26
2. PKG_RELEASE:=1 both to match the version in the PR of principal package: https-dns-proxy: update to 2023.12.26-1 packages#24715
3. You set the bootstrap servers to the IP addresses of the DOH resolvers as per: https://mullvad.net/en/help/dns-over-https-and-dns-over-tls#specifications. The bootstrap servers need to be the normal non-encrypted resolvers, unless Mullvad operates regular DNS resolvers I suggest you put back the old values.
4. Please rename the json file to net.mullvad.dns.json to match the new URL.
5. I'd appreciate it if you could create another json file for regional Mullvad servers (as per https://mullvad.net/en/help/dns-over-https-and-dns-over-tls#server-locations) called net.mullvad.json with "title": "Mullvad Regional",.

[Hekkfern]

Hi @stangri. Thanks for your feedback
As you requested, I pushed all the changes.

[stangri]

Thank you again @Hekkfern !