Bump dart bridge version to 1.7.1

1.7.1 is the first dart_bridge release whose dart_bridge.framework slices are
signed, not just the outer xcframework -- the counterpart of the change on this
branch for the Python XCFrameworks (flet-dev/dart-bridge#14).

Both halves have to move together. An IPA that picks up a 1.7.0 dart_bridge
alongside inner-signed Python XCFrameworks would still carry one receipt
reporting isSecureTimestamp = false, which leaves the ITMS-91065 question
unanswered rather than answered.

Version-keyed consumer caches hold the outer-only-signed 1.7.0 zip, so this must
be a version bump rather than a re-release. Compiled binaries are unchanged from
1.7.0 on every platform.

Author: FeodorFitsner

manifest.json

@@ -1,6 +1,6 @@
 {
   "default_python_version": "3.14",
-  "dart_bridge_version": "1.7.0",
+  "dart_bridge_version": "1.7.1",
   "pythons": {
     "3.12": {
       "full_version": "3.12.13",