Refactor CI ProjectConfig sources to use static precedence

What does this MR do and why?

This change refactors sources for ProjectConfig to use a static list of sources instead of manipulating it in EE to make the precedence of sources clearer.

EE-only configs return nil for CE.

The MR seems quite big, but it's mostly just moving existing code around.

MR acceptance checklist

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Related to #466430 (closed)

added Category:Security Policy Management backend devopsgovern groupsecurity policies maintenancerefactor sectionsec severity2 typemaintenance labels

assigned to @mcavoj

added pipelinetier-1 label

added 1 commit

• 943f3238 - Refactor CI ProjectConfig sources to use static precedence

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	2 Warnings
	This merge request is quite big (972 lines changed), please consider splitting it into multiple merge requests.
	The master pipeline status page reported failures in rspec unit pg14 7/44 If these jobs fail in your merge request with the same errors, then they are not caused by your changes. Please check for any on-going incidents in the incident issue tracker or in the #master-broken Slack channel.

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Category	Reviewer	Maintainer
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~"Verify"	Reviewer review is optional for ~"Verify"	@ayufan (UTC+2, 1 hour behind author)

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changed milestone to %17.3

added 1 commit

• 8f388fa3 - Use let_it_be for compliance project creation

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resolved all threads

added 1 commit

• 8e266661 - Refactor CI ProjectConfig sources to use static precedence

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Hi @furkanayhan, this MR addresses Fabio's suggestion here. Would you have time to review it?

The MR seems quite big but it's mostly just moving existing code around. If you prefer to split it up, I could move each config separately which could simplify the reviews.

requested review from @furkanayhan

mentioned in issue #471726 (closed)

removed review request for @furkanayhan

requested review from @Andyschoenen

approved this merge request

requested review from @fabiopitino and removed review request for @Andyschoenen