Clarify that use backup means restore

"Use key backup" feels like it's telling me I haven't set it up at all. This
changes to "Restore ..." which seems more reassuring.

Fixes element-hq/element-web#9438

src/components/views/settings/KeyBackupPanel.js

@@ -176,7 +176,7 @@ export default class KeyBackupPanel extends React.PureComponent {
         } else if (this.state.backupInfo) {
             const EmojiText = sdk.getComponent('elements.EmojiText');
             let clientBackupStatus;
-            let restoreButtonCaption = _t("Restore from Backup");
+            const restoreButtonCaption = _t("Restore from Backup");
 
             if (MatrixClientPeg.get().getKeyBackupEnabled()) {
                 clientBackupStatus = <div>
@@ -192,7 +192,6 @@ export default class KeyBackupPanel extends React.PureComponent {
                     )}</p>
                     <p>{_t("Back up your keys before signing out to avoid losing them.")}</p>
                 </div>;
-                restoreButtonCaption = _t("Use key backup");
             }
 
             let uploadStatus;

src/i18n/strings/en_EN.json

@@ -469,7 +469,6 @@
     "This device is backing up your keys. ": "This device is backing up your keys. ",
     "This device is <b>not backing up your keys</b>.": "This device is <b>not backing up your keys</b>.",
     "Back up your keys before signing out to avoid losing them.": "Back up your keys before signing out to avoid losing them.",
-    "Use key backup": "Use key backup",
     "Backing up %(sessionsRemaining)s keys...": "Backing up %(sessionsRemaining)s keys...",
     "All keys backed up": "All keys backed up",
     "Backup has a signature from <verify>unknown</verify> device with ID %(deviceId)s.": "Backup has a signature from <verify>unknown</verify> device with ID %(deviceId)s.",