The test test_multiple_unpublished_cdc_generations reads the CDC
generation timestamps to verify they are published in the correct order.
To do so it issues reads in a loop with a short sleep period and checks
the differences between consecutive reads, assuming they are monotonic.
However the assumption that the reads are monotonic is not valid,
because the reads are issued with consistency_level=ONE, thus we may read
timestamps {A,B} from some node, then read timestamps {A} from another
node that didn't apply the write of the new timestamp B yet. This will
trigger the assert in the test and fail.
To ensure the reads are monotonic we change the test to use consistency
level ALL for the reads.
Fixesscylladb/scylladb#24262Closesscylladb/scylladb#24272
