Fix quickly switching notification filters resulting in empty or incorrect list ...

Fix quickly switching notification filters resulting in empty or incorrect list (#19052)

Follow-up to #18960

The aforementioned PR fixed an issue in which switching notification filters
while notifications were loading prevented the query for the new filter from
running, but another issue remained: if the first query completed after the
second one, its results would override the second one, thus leading to the
same issue.

This commit cancels the first request if it is still running, before issuing
the second one.