[BUGS-5880] Adjust for multisite checks

[jazzsequence]

This PR switches the site to the DOMAIN_CURRENT_SITE if the site is multisite so that checks are run against the main/primary site. This effectively does the same thing that the --url=<primary site url> flag does in WP-CLI.

This had merged #141 but those changes have been backed out in 57f440a

• The majority of this change is here
• An additional change was needed in the insecure check to not warn about wp-redis making insecure function calls.
• The workflow was modified to not test older versions of PHP than 7.4 (8.0+ was tested but those tests failed, so updating was abandoned).
• Behat tests were updated to fix failing checks

[pwtyler]

LGTM, just one vestigial comment to drop.

[pwtyler]

Suggested change

# TODO Remove the version flag.