Fix memory leak in CI config includes entry

What does this MR do and why?

This backports !122462 (merged) to 16-0-stable-ee.

In GitLab 11.9, gitlab-foss!24098 (merged) introduced validation of include keywords. However, it quietly introduced a memory leak: any time a new include entry was instantiated, a proc would be added to the aspects class variable. As the list grew, the time taken to process other include keywords would grow.

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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