Support DoH3 and DoH2 under the same HTTPS URL with prioritisation

[MineGene]

Issue Details

Currently, AdGuard DNS serves DoH3 (h3://) and DoH2 (https://) under separate URLs. This requires users to choose between the two protocols manually, rather than allowing automatic fallback. Other DNS providers have started offering both protocols under the same https:// URL, enabling clients to negotiate the best available option.

Proposed solution

AdGuard DNS should support both DoH3 and DoH2 under the https:// URL, allowing clients to use DoH3 where supported and falling back to DoH2 when necessary. The prioritisation should be handled by the AdGuard DNS server, ensuring seamless client compatibility without requiring configuration changes.

Alternative solution

No response