Cleaning up codebase (WIP)

[nmolham-godaddy]

Summary

Cleaning up and optimize the codebase.

Details

There are so many aspects the codebase can be cleaned, so I thought about opening a PR to get feedback and inputs.

QA

• Code review
• Checks pass

Before merge

• I have confirmed these changes in each supported minor WooCommerce version

[agibson-godaddy]

I'm fine with these changes in theory, but I'm just "requesting changes" here because I think this can only go into a major release.

These updates are to abstract classes, which we extend and override in plugins. Just doing a normal composer update would immediately break those classes, e.g.

All the overridden methods (and properties) would have mismatching signatures, which will require manual work to update.

So I'm okay with it, but I think it has to be put in a major release like v6.0.0.

[nmolham-godaddy]

@agibson-godaddy Agreed, these changes are are considering to be breaking changes. I was also thinking about raising the PHP min version to PHP 8.0, but I realised that would be unrealistic considering WooCommerce requirements.