Switch to secure random strings (#29013)

* Switch to secure random strings

Because the js-sdk methods are changing and there's no reason for these
not to use the secure versions. The dedicated upper/lower functions were
*only* used in this one case, so this should do the exact same thing with
the one exported function.

Requires https://github.com/matrix-org/matrix-js-sdk/pull/4621 (merge both together)

* Change remaining instances of randomString

which I somehow entirely missed the first time.

* Fix import order

test/unit-tests/components/views/settings/Notifications-test.tsx

@@ -23,7 +23,7 @@ import {
     IThreepid,
     ThreepidMedium,
 } from "matrix-js-sdk/src/matrix";
-import { randomString } from "matrix-js-sdk/src/randomstring";
+import { secureRandomString } from "matrix-js-sdk/src/randomstring";
 import {
     act,
     fireEvent,
@@ -287,7 +287,7 @@ describe("<Notifications />", () => {
 
     beforeEach(async () => {
         let i = 0;
-        mocked(randomString).mockImplementation(() => {
+        mocked(secureRandomString).mockImplementation(() => {
             return "testid_" + i++;
         });