qa/crontab: use historically normal priorities for nightlies

qa/crontab: use historically normal priorities for nightlies

Stop using --force-priority except when necessary.

Squid still gets elevated priority due to the increased attention with the
imminent release.

I've differentiated the priorities some in that release branches should get
higher priority than the main branch and that older release branches should be
prioritized over newer ones. Finally, upgrade tests should be prioritized over
other nightlies.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>