chore!: namespace `Psr` library with Strauss

[justlevine]

What

This PR namespaces the Psr library, so it is now WPGraphQL/Login/Vendor/Psr/...

Why

In line with latest versions of strauss.

How

Testing Instructions

Additional Info

While this is technically a breaking change, it only affects internal code, and does not require changes in downstream projects.

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [x] I have added unit tests to verify the code works as intended.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

coverage: 82.571%. remained the same when pulling c5388b61ae0a01722473fbd8370963308824e90e on justlevine:chore/namespace-Psr into 923524fcf0d6462157a386d784650f847db3d9b8 on AxeWP:develop.