Bump unix's version number to 2.8.0.0 in HEAD

In order to perform a unix submodule bump (as requested in #15018 (closed)), we'll need to deal with the fact that its major version number was bumped upstream to 2.8.0.0. As a result, the following boot libraries will need to be adjusted:

• Cabal and cabal-install (ready, https://github.com/haskell/cabal/pull/5264)
• directory (ready, https://github.com/haskell/directory/pull/81)
• haskeline (ready, https://github.com/judah/haskeline/pull/82)
• iserv-bin (ready, in-place)
• process (ready, https://github.com/haskell/process/pull/126)

Some of these can be changed in-place, while others will require downstream changes. I'll update this ticket as I progress.