Set Asciidoctor outfilesuffix to default .adoc

What does this MR do?

In Asciidoc inter-document cross reference is generated with syntax <<document-b.adoc#section-b,Section B>>. By default Asciidoctor generates link to document-b.html. After this MR link is generated to document-b.adoc.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Without this MR, user needs to write conditions in document to make references work as expected in browser/GitLab and when running asciidoctor from terminal.

Example condition: ifdef::env-gitlab,env-browser[:outfilesuffix: .adoc]

Quote from http://asciidoctor.org/docs/user-manual/#navigating-between-source-files:

This configuration is no longer necessary on GitHub since GitHub now sets the value of outfilesuffix to match the file extension of the source file. However, it’s still required in GitHub-like environments such as GitLab.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#43192, gitlab-org/gitlab-ce#17602.