Remove Psr\Cache

[MGatner]

Description
Per the previous discussion, this removes Psr\Cache so it can be created as a separate module. I've started work on this module (https://github.com/MGatner/cache) - we will need to decide where it lives. I lean towards https://github.com/codeigniter4 since it is explicitly an add-on component that should be in the same namespace, but I could also see arguments for https://github.com/codeigniter4projects

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[lonnieezell]

I'm all for the main codeigniter4 group myself.

[MGatner]

Test failure is due to the inevitable code coverage reduction.