F# 6, 7 compatibility

We're getting both internal and external reports about missing F# 6 and 7 compatibility since the latest Rider release because of https://github.com/dotnet/fsharp/pull/19143.

The older language versions support was removed too abruptly for our users, as they weren't notified in advance and couldn't prepare for it accordingly.

Looking at the changes it seems that the removal didn't bring much value as is and at the same time it has definitely made it worse for users who weren't prepared for that.

I propose we:
* revert the language version support removal or improve how we handle projects using older versions
* add a warning about a (possible) future removal when an older language version is used
* decide on a date when/if it's going to be done (and give at least one year notice to the users)