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/useCallViewKeyboardShortcuts.ts

@@ -35,7 +35,7 @@ export function useCallViewKeyboardShortcuts(
   toggleMicrophoneMuted: () => void,
   toggleLocalVideoMuted: () => void,
   setMicrophoneMuted: (muted: boolean) => void
-) {
+): void {
   const spacebarHeld = useRef(false);
 
   // These event handlers are set on the window because we want users to be able