Add Log4J error to mod imcompatibilities

[dualspiral]

Not strictly the right place for it, but a place people look a lot.

[dualspiral]

This needs to target API-7, not API-8. Pretty sure API-8 needs a different version of Forge (36.2.20+)

Forgive me for the rambling, but there is are important reasons why I think it should still target the stable branch, as well as the API 7 branch (if there is one).

The ultimate problem is that we're still linking to this page, even though it's now "API 8". When I went and did a first pass tagging everything, I opted to change the page to say "SpongeAPI 7" specifically because if we blanked this page at this time, it would cause confusion. Additionally, we're still linking to the "stable" version of the page in other places, and I'm sure others are too.

Server owners who are trying to set up a server and look at the docs are also greeted with a large page with flags for languages. Nowhere is there an indication that these docs are for API-8. Because of how large it is, for most, it's not obvious there are old versions, and many new server owners won't necessarily link 7.4.0 to Minecraft 1.12.2. Let's face it, in general, server owners are just going to want to read some docs - and if they go to the incompatibilities page and can't see anything relavent, they're not necessarily going to know that they're looking at the wrong API version.

I think there is probably a larger conversation, outside of this ticket, about all of this. However, the above is why I updated that page with a header when I went through the pages, rather than wiping it.

[Inscrutable]

I have started this discussion in the Spongedocs discord channel so we can get some more input. My preference is to put a nice banner near the top, linking the Minecraft 1.12.2 (API-7) page in nice friendly words. API-8 has its own incompatibilities already and we need somewhere to put them.