TUN-9858: Remove proxy-dns feature from cloudflared

Remove the DNS over HTTPS (DoH) proxy feature built on CoreDNS due to
security vulnerabilities (GO-2025-3942, GO-2026-4289).

This removes:
- Standalone proxy-dns command (cloudflared proxy-dns)
- Tunnel subcommand (cloudflared tunnel proxy-dns)
- Proxy-dns flags for tunnel run (--proxy-dns, --proxy-dns-port, etc.)
- Config file resolver section support
- tunneldns/ package (CoreDNS-based implementation)
- Related component tests

BREAKING CHANGE: The proxy-dns feature is no longer available.
Users should migrate to alternative DNS over HTTPS solutions.

CHANGES.md

@@ -1,3 +1,14 @@
+## 2026.2.0
+### Breaking Change
+- Removes the `proxy-dns` feature from cloudflared. This feature allowed running a local DNS over HTTPS (DoH) proxy.
+  Users who relied on this functionality should migrate to alternative solutions.
+  
+  Removed commands and flags:
+  - `cloudflared proxy-dns`
+  - `cloudflared tunnel proxy-dns` 
+  - `--proxy-dns`, `--proxy-dns-port`, `--proxy-dns-address`, `--proxy-dns-upstream`, `--proxy-dns-max-upstream-conns`, `--proxy-dns-bootstrap`
+  - `resolver` section in configuration file
+
 ## 2025.7.1
 ### Notices
 - `cloudflared` will no longer officially support Debian and Ubuntu distros that reached end-of-life: `buster`, `bullseye`, `impish`, `trusty`.