v1.7.3: drop tun2proxy fork dependency (#271)

Move from yyoyoian-pixel/tun2proxy fork (with patched JNI signature)
to canonical tun2proxy 0.7.21 from crates.io with feature flag
"udpgw". Cargo.toml [patch.crates-io] section removed entirely.

The Android side now resolves tun2proxy_run_with_cli_args at runtime
via dlsym from libtun2proxy.so, which is the upstream maintainer's
recommended path for callers that need full CLI flexibility.
mhrv-rs builds the CLI string in MhrvVpnService and passes it through
Native.runTun2proxy → src/android_jni.rs → dlsym → tun2proxy.

Future tun2proxy upgrades are now a single Cargo version bump.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2222,7 +2222,7 @@ dependencies = [
 
 [[package]]
 name = "mhrv-rs"
-version = "1.7.2"
+version = "1.7.3"
 dependencies = [
  "base64 0.22.1",
  "bytes",