Release 6.6.6

Maintenance release with development tooling updates. No changes for site admins or form users.

Dev Upgrades

• Code style checks: upgraded PHP_CodeSniffer to 3.13.6 and WordPress Coding Standards to 3.4.1, which clears two security alerts on these dev-only tools.
• CI and release workflows: updated GitHub's checkout action to v6.

Full Changelog: 6.6.5...6.6.6