Do not build universum-doctest with GHC >= 9.0.0

Problem: universum-doctest passes with GHC < 9, but starts to
fail with >= 9. The errors are weird, it complains that a lot of
stuff is not in scope. Example:
> Not in scope: type constructor or class ‘Text’

Solution: there is the following issue
sol/doctest#327. Apparently, something is
wrong in `doctest` itself. The maintainer intends to investigate it,
so let's wait for that. While waiting, let's disable
`universum-doctest` whenever GHC is >= 9.0.0.

universum.cabal

@@ -149,6 +149,10 @@ test-suite universum-doctest
 
   if impl(ghc >= 8.10.1)
     ghc-options:         -Wno-missing-safe-haskell-mode
+  -- https://github.com/sol/doctest/issues/327
+  -- TODO: re-enable when the issue is resolved
+  if impl(ghc >= 9.0.0)
+    buildable: False
 
   ghc-options:         -threaded