Update SecureBackupPanel to use getCrypto() (#11322)

`MatrixClient.crypto` is going away, so let's switch over to `getCrypto()`.

There doesn't seem to be anything else relying on the `crypto` stub in
`mockClientMethodsCrypto()`, so let's get rid of that.

src/components/views/settings/SecureBackupPanel.tsx

@@ -146,7 +146,7 @@ export default class SecureBackupPanel extends React.PureComponent<{}, IState> {
 
     private async getUpdatedDiagnostics(): Promise<void> {
         const cli = MatrixClientPeg.safeGet();
-        const crypto = cli.crypto;
+        const crypto = cli.getCrypto();
         if (!crypto) return;
 
         const secretStorage = cli.secretStorage;