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/libpsl/package.py

@@ -12,7 +12,7 @@ class Libpsl(AutotoolsPackage):
     homepage = "https://github.com/rockdaboot/libpsl"
     url      = "https://github.com/rockdaboot/libpsl/releases/download/libpsl-0.17.0/libpsl-0.17.0.tar.gz"
 
-    version('0.17.0', 'fed13f33d0d6dc13ef24de255630bfcb')
+    version('0.17.0', sha256='025729d6a26ffd53cb54b4d86196f62c01d1813a4360c627546c6eb60ce3dd4b')
 
     depends_on('icu4c')