Support MSC4287 m.key_backup stable prefix as well as the unstable prefix (#33034)

* Support MSC4287 m.key_backup stable prefix as well as the unstable prefix

* Update comments to avoid talking about EX since this now applies more widely

* Make comments about m.key_backup more helpful

* Improve comment on recheckBackupDisabled

* Explicitly say backup is disabled only if enabled is actually set to false

apps/web/test/unit-tests/components/viewmodels/settings/encryption/KeyStoragePanelViewModel-test.ts

@@ -128,6 +128,10 @@ describe("KeyStoragePanelViewModel", () => {
         expect(mocked(matrixClient.setAccountData)).toHaveBeenCalledWith("m.org.matrix.custom.backup_disabled", {
             disabled: false,
         });
+
+        expect(mocked(matrixClient.setAccountData)).toHaveBeenCalledWith("m.key_backup", {
+            enabled: true,
+        });
     });
 
     it("should delete key storage when disabling", async () => {
@@ -145,5 +149,8 @@ describe("KeyStoragePanelViewModel", () => {
         expect(mocked(matrixClient.setAccountData)).toHaveBeenCalledWith("m.org.matrix.custom.backup_disabled", {
             disabled: true,
         });
+        expect(mocked(matrixClient.setAccountData)).toHaveBeenCalledWith("m.key_backup", {
+            enabled: false,
+        });
     });
 });