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/CallEndedView.tsx

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { FC, FormEventHandler, useCallback, useState } from "react";
+import { FC, FormEventHandler, ReactNode, useCallback, useState } from "react";
 import { MatrixClient } from "matrix-js-sdk/src/client";
 import { Trans, useTranslation } from "react-i18next";
 import { useHistory } from "react-router-dom";
@@ -148,7 +148,7 @@ export const CallEndedView: FC<Props> = ({
     </div>
   );
 
-  const renderBody = () => {
+  const renderBody = (): ReactNode => {
     if (leaveError) {
       return (
         <>