Second batch: remove deprecated calls on MatrixClient (#28227)

* Replace `MatrixClient.forceDiscardSession` call by `MatrixClient.CryptoApi.forceDiscardSession`

* Remove `MatrixClient.scheduleAllGroupSessionsForBackup` mock

src/SlashCommands.tsx

@@ -727,7 +727,7 @@ export const Commands = [
         isEnabled: (cli) => !isCurrentLocalRoom(cli),
         runFn: function (cli, roomId) {
             try {
-                cli.forceDiscardSession(roomId);
+                cli.getCrypto()?.forceDiscardSession(roomId);
             } catch (e) {
                 return reject(e instanceof Error ? e.message : e);
             }