Switch to secure random strings (#29013) (#29035)

* 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

(cherry picked from commit 56eafc90)

Co-authored-by: David Baker <dbkr@users.noreply.github.com>