Upgrade eslint-plugin-matrix-org to 1.2.1

This upgrade came with a number of new lints that needed to be fixed across the code base. Primarily: explicit return types on functions, and explicit visibility modifiers on class members.

src/room/useJoinRule.ts

@@ -15,12 +15,14 @@ limitations under the License.
 */
 
 import { useCallback } from "react";
+import { JoinRule } from "matrix-js-sdk/src/matrix";
 
 import type { Room } from "matrix-js-sdk/src/models/room";
 import { useRoomState } from "./useRoomState";
 
-export const useJoinRule = (room: Room) =>
-  useRoomState(
+export function useJoinRule(room: Room): JoinRule {
+  return useRoomState(
     room,
     useCallback((state) => state.getJoinRule(), [])
   );
+}