SecureBackupPanel: stop using deprecated APIs, and other fixes (#11644)

* SecureBackupPanel: replace `isKeyBackupTrusted`

`MatrixClient.isKeyBackupTrusted` -> `CryptoApi.isKeyBackupTrusted`

* SecureBackupPanel: replace `getKeyBackupEnabled`

`MatrixClient.getKeyBackupEnabled` -> `CryptoApi.getActiveSessionBackupVersion`

* SecureBackupPanel: replace `deleteKeyBackupVersion`

`MatrixClient.deleteKeyBackupVersion` -> `CryptoApi.deleteKeyBackupVersion`

* Do not show session count if we have no info

We shouldn't say "zero sessions to back up" if we don't know.

* SecureBackupPanel: distinguish between server and active backup

test/components/views/settings/__snapshots__/SecureBackupPanel-test.tsx.snap

@@ -140,20 +140,27 @@ exports[`<SecureBackupPanel /> suggests connecting session to key backup when ba
         <th
           scope="row"
         >
-          Backup version:
+          Latest backup version on server:
         </th>
         <td>
           1
+           (
+          Algorithm:
+           
+          <code>
+            test
+          </code>
+          )
         </td>
       </tr>
       <tr>
         <th
           scope="row"
         >
-          Algorithm:
+          Active backup version:
         </th>
         <td>
-          test
+          None
         </td>
       </tr>
     </table>