build: Upgrade to GutenbergKit v0.15.0 (#25479)

Integrate the latest features and bug fixes.

Author: dcalhoun

Modules/Package.resolved

@@ -55,7 +55,7 @@ let package = Package(
         .package(url: "https://github.com/wordpress-mobile/wpxmlrpc", from: "0.9.0"),
         .package(url: "https://github.com/wordpress-mobile/NSURL-IDN", revision: "b34794c9a3f32312e1593d4a3d120572afa0d010"),
         .package(url: "https://github.com/zendesk/support_sdk_ios", from: "8.0.3"),
-        .package(url: "https://github.com/wordpress-mobile/GutenbergKit", from: "0.14.0"),
+        .package(url: "https://github.com/wordpress-mobile/GutenbergKit", from: "0.15.0"),
         // We can't use wordpress-rs branches nor commits here. Only tags work.
         .package(url: "https://github.com/Automattic/wordpress-rs", revision: "alpha-20260313.1"),
         .package(