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

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