Update PHPStan level to 6 and fix violations and bump minimum PHP to 7.2

[felixarntz]

This PR performs some updates to improve code quality and modernize the codebase a little:

• Update PHPStan level to 6 and fix all the violations it flags (which lead to better type safety).
• Bump minimum PHP version required to 7.2 (there's no good reason to support 7.0, and e.g. void return type doesn't work on 7.0).
• Update Composer dependencies.
• Use latest versions in GitHub workflows.

[adamsilverstein]

Spiffy!