checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/at-spi2-core/package.py

@@ -16,7 +16,7 @@ class AtSpi2Core(MesonPackage):
     list_url = "http://ftp.gnome.org/pub/gnome/sources/at-spi2-core"
     list_depth = 1
 
-    version('2.28.0', '9c42f79636ed1c0e908b7483d789b32e')
+    version('2.28.0', sha256='42a2487ab11ce43c288e73b2668ef8b1ab40a0e2b4f94e80fca04ad27b6f1c87')
 
     depends_on('glib@2.56.1:')
     depends_on('dbus@1.12.8:')