Add APCu to 'always installed' list

[fredden]

Purpose of this pull request

This pull request adds APCu to the list of PHP extensions that are always installed on Magento Cloud. This is a feature request that is being discussed in magento/ece-tools#795

Affected DevDocs pages

• https://devdocs.magento.com/cloud/project/magento-app-php-application.html

[BarnyShergold]

@fredden - There hasn't been any movement on this since the Draft was created nearly 3 weeks ago - please can we have an update on progress?

[fredden]

This is waiting on an infrastructure change that only Adobe can make. magento/ece-tools#795 (comment) I've enquired with Adobe staff on the Magento Community Engineering Slack workspace and they've confirmed that Adobe is unwilling to grant access to the relevant repositories for me to help further.

I don't envisage the changes here needing any further work (except resolving conflicts), but it doesn't make sense to merge this before the infrastructure change has been made. You're welcome to click the 'Ready for review' button here if you'd like.

[dobooth]

We want to encourage doc contributions in tandem with code contributions. Getting things through the code base is slightly more complex than devdocs, so I think we can be patient with this draft. Thanks for the work.

[dobooth]

@fredden I will contradict myself and vote to close this for now. It's a simple enough reopen it when the ece-tools PR goes through.

[ghost]

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