[new-backend] Change system:/elektra/mountpoints and system:/elektra/modules/<plugin name>/exports to singular

[kodebach]

system:/elektra/mountpoints

This should be changed to system:/elektra/mountpoint as we have in doc/DESIGN.md "We use singular for all names.".

Originally posted by @markus2330 in #4442 (comment)

[atmaxinger]

Also system:/elektra/modules/<plugin name>/exports

[github-actions]

I mark this stale as it did not have any activity for one year. I'll close it in two weeks if no further activity occurs. If you want it to be alive again, ping by writing a message here or create a new issue with the remainder of this issue.
Thank you for your contributions 💖

[github-actions]

I closed this now because it has been inactive for more than one year. If I closed it by mistake, please do not hesitate to reopen it or create a new issue with the remainder of this issue.
Thank you for your contributions 💖