Remove tests that only ran on PHP 7 and are always skipped on PHP 8.1+ (#1986)

PHP 8.1 is the minimum supported version. Tests with
skip_if_php_version('>=', '7.99') are permanently skipped and can be
removed. Their PHP 8 counterparts (where applicable) remain in place.

Also remove the now-unnecessary skip_if_php_version('<', '7.99') from
session-startTransaction_error-005.phpt, which always runs on PHP 8.1+.

Replace skip_if_php_version('>=', '8.1.99') with the explicit '8.2'
version in bug1598-002 tests; '8.1.99' was used as a pre-release
placeholder when PHP 8.2 was not yet released.

Author: GromNaN