[docs] Change notice for versions of Moodle only in security support

[ericmerrill]

What is the URL of the page?

http://localhost:3000/general/releases/4.5/4.5.13

What is the issue with this page?

Moodle versions that are only in security support have this banner:

This incorrectly states that the version of Moodle is unsupported, and they should upgrade to a supported version - which is incorrect.

Are you able to provide a patch for this?

Yes - I will create one

[ericmerrill]

I've attached a PR that also has a screenshot of the change. As mentioned there, I'm not tied to the particular wording if people want to suggest changes.