from ghc-9.2 on, the “default” langauge of GHC is expected to change
more wildly. To prepare for that (and unblock
ghc/ghc!4853), this sets the
language for all the test files to `Haskell2010`. This should insolate
this test suite against changes to the default.

Cherry-picked from https://github.com/haskell/haddock/pull/1341