Revert "Update Cabal submodule to HEAD (1.21)"

This reverts commit af28e61c.
We'll wait until it's possible to bootstrap from 7.6 out of the box.

Cabal @ c125342d

-Subproject commit 337f9cd7927b787c6796acddc943393cf5b8e64c
+Subproject commit c125342d4147ffb59c88d43024ae9abfc3a9c96d

libraries/bin-package-db/bin-package-db.cabal

@@ -26,5 +26,5 @@ Library
 
     build-depends: base >= 4 && < 5,
                    binary >= 0.5 && < 0.8,
-                   Cabal >= 1.20 && < 1.22
+                   Cabal >= 1.20 && < 1.21
 

utils/ghc-cabal/ghc-cabal.cabal

@@ -18,7 +18,7 @@ Executable ghc-cabal
 
     Build-Depends: base       >= 3   && < 5,
                    bytestring >= 0.10 && < 0.11,
-                   Cabal      >= 1.20 && < 1.22,
+                   Cabal      >= 1.20 && < 1.21,
                    directory  >= 1.1 && < 1.3,
                    filepath   >= 1.2 && < 1.4
 

utils/ghctags/ghctags.cabal

@@ -19,6 +19,6 @@ Executable ghctags
 
     Build-Depends: base       >= 4   && < 5,
                    containers,
-                   Cabal      >= 1.20 && <1.22,
+                   Cabal      >= 1.20 && <1.21,
                    ghc