Publish the feature - Limit pipeline job concurrency using named semaphores

What does this MR do?

There is an upstream MR !21899 (merged)

This MR publishes the feature Limit pipeline job concurrency using named semaphores as GitLab 12.7 release. Specifically, it does the following things:

• Make the feature flag ci_resource_group enabled by default
• Add documentation about how to use resource_group key.
• Add changelog

Closes #15536 (closed)

Related issues

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