Add PHP 8-only Support (v2)

[patinthehat]

This PR adds a new major version, v2.0.0.

Specifically, it:

• Removes support for all PHP 7.x versions.
• Requires PHP 8.0+.
• All syntax converted to PHP 8 where possible.
• Removes unnecessary PHP docblocks per Spatie's guidelines.
• Removes deprecated methods.
• Migrates from StyleCI to php-cs-fixer + github workflow for styling to match other Spatie packages.
• Adds several missing files (FUNDING.yml, SECURITY.md, etc.) that are standard in other Spatie packages.
• Updates the changelog - release date needs to be updated, currently is "unreleased".

[freekmurze]

Thank you!