Build GHC in GHC2021

Dear GHC devs, Year 2024 is upon us. GHC2021 was released 2 years ago, in GHC 9.2. I think it's time for the base language for GHC to be GHC2021. What do we think? PS: I don't actually know where the base language for GHC is specified. I couldn't find it with a cursory search PPS: if someone makes an MR, bonus points for clearing up hundreds of lines of now-unnecessary extension on top of files. -- Arnaud Spiwack Director, Research at https://moduscreate.com and https://tweag.io.