Site Health: If there is only one recommendation, use singular

[Presskopp]

Having 0 critical issues but 1 recommendation you can read on the dashboard:

Your site’s health is looking good, but there are still some things you can do to improve its performance and security.

We should rather say:

Your site’s health is looking good, but there is still a thing you can do to improve its performance and security.

[Hareesh Pillai]

While we are at it, it is meaningful if we also address the case where there is just one critical issue.

Presently it is:

Your site has critical issues that should be addressed as soon as possible to improve its performance and security.

Instead it should be

Your site has a critical issue that should be addressed as soon as possible to improve its performance and security.

[SergeyBiryukov]

In 50530:

Site Health: Use a singular string in dashboard widget if there is only one critical issue or recommendation.

Props Presskopp, hareesh-pillai.
Fixes #52521.