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.

component-tests/config.py

@@ -5,7 +5,7 @@ import base64
 
 from dataclasses import dataclass, InitVar
 
-from constants import METRICS_PORT, PROXY_DNS_PORT
+from constants import METRICS_PORT
 
 # frozen=True raises exception when assigning to fields. This emulates immutability
 
@@ -99,10 +99,3 @@ class QuickTunnelConfig(BaseConfig):
         object.__setattr__(self, 'full_config',
                            self.merge_config(additional_config))
 
-@dataclass(frozen=True)
-class ProxyDnsConfig(BaseConfig):
-    full_config = {
-        "port": PROXY_DNS_PORT,
-        "no-autoupdate": True,
-    }
-