Numerous tests make use of CUSKs (complete user-supplied kinds),
a legacy feature scheduled for deprecation. In order to proceed
with the said deprecation, the tests have been updated to use SAKS
instead (standalone kind signatures).

This also allows us to remove the Haskell2010 language pragmas that
were added in 115cd3c8 to work around the lack of CUSKs in GHC2021.