Enable key backup by default

When we set up cross signing, so the key backup key will be stored locally along with the cross signing keys until the user sets up recovery (4s). This will mean that a user can restore their backup if they log in on a new device as long as they verify with the one they registered on.

Replaces https://github.com/element-hq/element-web/pull/28267

test/unit-tests/components/structures/MatrixChat-test.tsx

@@ -1003,6 +1003,7 @@ describe("<MatrixChat />", () => {
                     userHasCrossSigningKeys: jest.fn().mockResolvedValue(false),
                     // This needs to not finish immediately because we need to test the screen appears
                     bootstrapCrossSigning: jest.fn().mockImplementation(() => bootstrapDeferred.promise),
+                    resetKeyBackup: jest.fn(),
                     isEncryptionEnabledInRoom: jest.fn().mockResolvedValue(false),
                 };
                 loginClient.getCrypto.mockReturnValue(mockCrypto as any);