Prepare an unexpected 2.23.1 release

Unfortunately, the 2.23.0 release is broken on 8.8.2 and 8.8.3. However,
there is no good backwards compatible fix. The fix going forward is:

  - Revise already release `haddock-api-2.23.0` to build only on
    `ghc-8.8.1` (this is the truth - it won't build on 8.8.2 or 8.8.3)

  - Release `haddock-api-2.23.1` and `haddock-2.23.1` to build only on
    `ghc-8.8.2` and `ghc-8.8.3`

Given that there are hardly any commits between these two releases, and
this is mostly one (not user visible) backwards incompatible change,
the changelog won't be updated.