Compatibility issue between Forms 8.10 and CMS <= 8.7

[AndyButland]

If running Forms 8.10 and CMS 8.6 or less, forms aren't rendering correctly in the back-office.

This is happening due to the "content apps for forms" feature introduced in 8.10, which relies on a fix made in CMS 8.7 to allow for the display of content apps in sections other than content and media. Specifically the fix made in response to this comment, as part of this PR, which introduced content apps to the "Settings" section.

To fix we need to detect if CMS 8.6 or less is running, and if not, don't attempt to render content apps. This will avoid calling the code that triggers the NotSupportedException.

If customers of Forms 8 wish to use content apps in the forms section, they will need to be running at least CMS 8.7.