Define SSoT precedence of CI configurations in CE

The following discussion from !150334 (merged) should be addressed:

• @fabiopitino started a discussion:

  @mcavoj @furkanayhan I think it would be better to move a SSoT order of sources in CE, since we are also adding the sources enum in in CE.

  In CE source classes like PipelineExeuctionPoilicyForce#content can always return nil and will be extended in EE. This way the placeholder of the source in enums and ProjectConfig match.

  We also don't need to do any manipulations with sources like insert_before_autodevops. We can have a static precedence with clear comments on the why.