GHC 9.12.4 is now available

@wz1000 Thanks for the long awaited release! Would it make sense to delay the announcements of new GHC versions here until they become available with ghcup, at least in the vanilla branch? After all, ghcup is the most popular installation method, if we assign relevance to the survey State of Haskell 2025 results . Based on recent experience, it seems that it takes in average a week for the new version to get available with ghcup. It would be nice that when I read the announcement, I can directly crank up ghcup to deliver the new version to me.