build: track GutenbergKit trunk for native media uploads

Moves the pin off the pr-build/357 snapshot now that
wordpress-mobile/GutenbergKit#357 has merged. Trunk also carries the
follow-up hardening in #561, which adds a defaulted handlesFile(ofType:
named:) to MediaUploadDelegate, so GBKMediaUploadProcessor conforms
unchanged.

No tagged release includes #357 yet — v0.19.0 predates it. Swap to a
tagged release before merge.

Author: dcalhoun

Modules/Package.resolved

@@ -62,7 +62,7 @@ let package = Package(
             revision: "b34794c9a3f32312e1593d4a3d120572afa0d010"
         ),
         .package(url: "https://github.com/zendesk/support_sdk_ios", from: "8.0.3"),
-        .package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "pr-build/357"),
+        .package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "trunk"),
         .package(
             url: "https://github.com/automattic/wordpress-rs",
             exact: "0.6.0"