Fix misc. Github Action warning

[mfn]

Summary

Some of the the referenced actions are already quite outdated and cause a lot of warnings emmited if you check the workflow runs:

• bump actions/cache
  Fixes:

  Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/cache@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

• replace set-output with GITHUB_OUTPUT
  Fixes:

  The set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

• bump codecov/codecov-action
  Fixes

  `The following actions uses node12 which is deprecated and will be forced to run on node16: codecov/codecov-action@v1. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

  I checked the workflow and upload works, see https://github.com/PHP-Open-Source-Saver/jwt-auth/actions/runs/7986070882/job/21805731559?pr=232#step:11:50

• bump actions/checkout
  Fixes

  Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2, actions/cache@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Notes

• About the php-cs-fixer commit: this is unrelated to my changes, it simply wasn't run in a long time and php-cs-fixer evolved and therefore applies these changes.
• They are still warnings emmited, but they're of different nature:
  

[mfn]

Also changed directory to dir: streamlined in this repo and with online examples usually found

[Messhias]

@mfn looks very good/nice to me. If you feel good about it I can merge it this weekend.

Thanks :)

[mfn]

The tests are green, you can merge immediately :)