Add new GHC version and raise base bound

Author: JackKelly-Bellroy

Diff for: flake.nix

@@ -13,6 +13,7 @@
         "ghc96"
         "ghc98"
         "ghc910"
+        "ghc912"
       ];
       defaultCompiler = "ghc96";
     };

Diff for: wai-handler-hal.cabal

@@ -37,6 +37,7 @@ tested-with:
    || ==9.6.6
    || ==9.8.2
    || ==9.10.1
+   || ==9.12.1
 
 common opts
   default-language: Haskell2010
@@ -49,7 +50,7 @@ common opts
 
 common deps
   build-depends:
-    , base                  >=4.12      && <4.21
+    , base                  >=4.12      && <4.22
     , base64-bytestring     >=1.0.0.0   && <1.3
     , bytestring            >=0.10.8    && <0.13
     , case-insensitive      ^>=1.2.0.0