Alpha list styles

Add a custom "alpha" ordered list style type for the core/list block and increment the minimum required WordPress version and tested-up-to version.

Notable changes

Fixed

• ⚠️ PHP lint warnings.

Added

• Custom "alpha" style option for the core/list block (along with matching CSS rules).

Changed

• ⬆️ Upgrade npm and composer dependencies.
• Increment WP minimum version and tested-up-to version.
• Filter the post date display on single posts using a custom template tag.