build-aux: Update Flatpak libdatachannel to v0.21.1

This update needs to be done before the runtime change, otherwise the newer GCC
will fail to build it because of libdatachannel missing headers.

build-aux/modules/50-libdatachannel.json

@@ -14,8 +14,8 @@
             "type": "git",
             "url": "https://github.com/paullouisageneau/libdatachannel.git",
             "disable-submodules": true,
-            "tag": "v0.21.0",
-            "commit": "9d5c46b8f506943727104d766e5dad0693c5a223"
+            "tag": "v0.21.1",
+            "commit": "898bdffe7340134f0891633cc7a6f6d2132c31c3"
         }
     ]
 }