Extract repeated setup steps into reusable composite actions

[dottxado]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature

What is the current behavior? (You can also link to an open issue here)

Fixes #247
Fixes #183

What is the new behavior (if this is a feature change)?

• Extract three composite actions
• Incorporate composer validate on the setup-php composite action so that all the workflows calling it can benefit (before only a part of the workflows were validating composer files)
• Incorporate the package manager input in the setup-node so that, when we remove Yarn, the logic and the deprecation warning live only in the action
• Use kebab-case for the actions input, following GitHub convention

Out of scope:

• deprecated workflows have not been modified

Bonus:

• removed the "delete signing key" from the workflows doing that step: since I was working to extract the signing key logic, it made sense to me to execute that task as well
• composite actions pin third-party actions by their commit SHA ([Feature Request]: security: pin all third-party action references to commit SHAs #248)

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No, the interface of the workflows has not been changed.

Other information

Log of the tests

• automatic-release.yml log ✅
• build-and-distribute.yml log ✅
• coding-standards-php.yml log ✅
• deploy-deployer.yml log ✅
• lint-php.yml log ✅
• static-analysis-js.yml log ✅ (the workflow didn't end successfully for other reasons)
• static-analysis-php.yml log ✅
• test-playwright.yml log ✅
• tests-unit-js.yml log ✅
• tests-unit-php.yml log ✅
• wp-scripts-lint.yml log ✅

[tyrann0us]

Thank you very much for working on this! I really love how this reduces repetition big time. 💪🏽
There's still some room for improvement, IMO; I left several line comments. Also, I feel like AI desperately wants to write a "Notes" section at the end of each documentation file; do we really need those?

[tyrann0us]

Thank you for implementing all the changes. LGTM 💪🏽