Align setup-magento action with upstream defaults

[digitalrisedorset]

Use this:

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (not applicable)
• Docs have been added / updated (not applicable)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

The setup-magento action diverged from the upstream implementation:

• default PHP version was still set to 8.3
• default Magento version was outdated
• composer_auth input handling was missing
• COMPOSER_AUTH was not passed to composer-related steps

Fixes: N/A

What is the new behavior?

Align setup-magento/action.yml with upstream defaults by:

• bumping the default PHP version to 8.4
• updating the default Magento version
• adding composer_auth input support
• forwarding COMPOSER_AUTH to composer-related steps

Does this PR introduce a breaking change?

• Yes
• No

Other information

This change assumes environments are not relying on Traefik-specific routing behaviour.

[damienwebdev]

lgtm