JSON for Hash now has to be Base16

Fix #14532.

As discussed on the call today:

1. We'll stick with `format = "base16"` and `hash = "<hash>"`, not do
   `base16 = "<hash>"`, in order to be forward compatible with
   supporting more versioning formats.

   The motivation we discussed for someday *possibly* doing this is
   making it easier to write very slap-dash lang2nix tools that create
   (not consume) derivations with dynamic derivations.

2. We will remove support for non-base16 (and make that the default, not
   base64) in `Hash`, so this is strictly forward contingency, *not*
   yet something we support. (And also not something we have concrete
   plans to start supporting.)

src/json-schema-checks/meson.build

@@ -32,10 +32,8 @@ schemas = [
     'stem' : 'hash',
     'schema' : schema_dir / 'hash-v1.yaml',
     'files' : [
-      'sha256-base64.json',
       'sha256-base16.json',
-      'sha256-nix32.json',
-      'blake3-base64.json',
+      'blake3-base16.json',
     ],
   },
   {