Remove remaining references to .es6 file extension in our config files and documentation

This is a follow up to MR !9241 (merged) which removed all *.es6 file extensions. This MR removes *.es6 from all of our config files, comments, and documentation.

This should be pretty straight forward. Note that the changes to the frontend contribution docs will be handled by !9961 (merged)

• [ ] Changelog entry added
• Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

n/a