Remove known-stale projects from the open projects page

This removes or modifies:

Undefined behavior checking -- we've got UBSan and it's well-known
enough that we don't need to call it out specifically as a need.

Improve target support -- this is largely already complete.

Use clang libraries to extend Ragel with a JIT -- this wasn't really
related to improving Clang.

C++1y feature work -- now talks about C++20, C++2b, and C2x instead.

Universal driver -- the bug report linked to by the UniversalDriver
web page has been closed since 2017.

XML representation of the AST -- we removed this functionality in 2013
and replaced it with a JSON representation a few years later. That is a
best-effort project.