Update flatbuffers to 1.11.0

Previous release fails to build due to code errors detected with
newer toolchain.

deps/flatbuffers.json

@@ -3,8 +3,8 @@
     "sources": [
         {
             "type": "archive",
-            "url": "https://github.com/google/flatbuffers/archive/v1.10.0.tar.gz",
-            "sha256": "3714e3db8c51e43028e10ad7adffb9a36fc4aa5b1a363c2d0c4303dd1be59a7c"
+            "url": "https://github.com/google/flatbuffers/archive/v1.11.0.tar.gz",
+            "sha256": "3f4a286642094f45b1b77228656fbd7ea123964f19502f9ecfd29933fd23a50b"
         }
     ],
     "cleanup": [