Allow to use issue/issues and merge_request/merge_requests for issuable_type in Insights configuration

What does this MR do?

Sometimes people use issuable_type: issues instead of issuable_type: issue or issuable_type: merge_requests instead of issuable_type: merge_request.

This MR makes the Insights finder handle the plural form transparently without throwing an error.

Note:

• I haven't documented this new behavior because I see it as a "nice to have" but I think it's simpler to only document only one form (we could also decide to advertise the plural form if we think it makes more sense...).
• The finder specs are very heavy (they takes 2 minutes to run) since the setup handles a combination of issues/MRs for projects/groups. I didn't want to spend too much time optimizing it but I think that could become problematic if we keep adding new cases over time...

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #13005 (closed).