Bump/php7.4

[Mike-Hermans]

Issue link

Description

• Updates minimum PHP version to 7.4
• Updates Psalm from major version 3 to 4
• Fixes new Psalm issues
• Fixes for differences between \WP_Block_Type and \WP_Block renderer
• Throw exceptions instead of mixing return types between values and \WP_Error
• Fixes #229

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [X] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.