ZTS: fix io_uring test on RHEL 9 variants (#16411)

Simplify the test, by using the variable "$PLATFORM_ID" in favor
of "$REDHAT_SUPPORT_PRODUCT_VERSION".

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>