Change deprecation warnings into errors for 7.0

[bluetech]

Per our backward compatibility policy, change deprecations for pytest 7 to errors, before complete removal in pytest 7.1.

Note: @nicoddemus already opened a similar issue in #7363, but it was changed to track the work for pytest 8.0. I believe we can still do some removals in 7.0 -- (TODO: link to PR). Specifically deprecations from 6.0 are eligible, but no later.