Update DnsLibs to 2.7.6 in v4.5 branch

[ameshkov]

Issue Details

There have been many changes since v2.5.5 that is currently used by AdGuard for iOS:
https://github.com/AdguardTeam/DnsLibs/blob/master/CHANGELOG.md

The main feature is that v2.7 adds Post-Quantum cryptography support

Proposed solution

Updated DnsLibs to v2.7

Alternative solution

No response