Bump Cabal submodule to 3.0.0.0-rc3

Cabal @ c6ffad56

-Subproject commit cbb47e6cf03c578d7435ec1acb56440bd6ebb9d8
+Subproject commit c6ffad5662cf97b688c1e4a516d436b8f909f190

libraries/base/base.cabal

-cabal-version:  2.2
+cabal-version:  3.0
 name:           base
 version:        4.13.0.0
 -- NOTE: Don't forget to update ./changelog.md

libraries/ghc-heap/ghc-heap.cabal.in

-cabal-version:  2.1
+cabal-version:  3.0
 name:           ghc-heap
 version:        @ProjectVersionMunged@
 license:        BSD-3-Clause

rts/rts.cabal.in

-cabal-version: 2.5
+cabal-version: 3.0
 name: rts
 version: 1.0
 license: BSD-3-Clause