Update supported specification version to 1.0.28

I looked into all changes between our current version 1.0.19 and the
current version of the specification 1.0.28 and I agree with Jussi that
the only one not fully resolved is:
"8dafd00 (tag: v1.0.24) Clarify optional attributes" and more precisely
the changes from commit:
theupdateframework/specification@4dd279b

It doesn't make sense to have a target file without "paths" or
"path_hash_prefixes", so our `python-tuf requirement to have at least
one of them set makes sense.

Both with Jussi we agreed that we can easily loosen this requirement if
when solving theupdateframework/specification#200
it's decided that both of them can be omitted,
but for now, we decided it's better to stick to our current requirement
to have one of them set.

Signed-off-by: Martin Vrachev <[email protected]>

tuf/api/metadata.py

@@ -74,7 +74,7 @@
 
 # We aim to support SPECIFICATION_VERSION and require the input metadata
 # files to have the same major version (the first number) as ours.
-SPECIFICATION_VERSION = ["1", "0", "19"]
+SPECIFICATION_VERSION = ["1", "0", "28"]
 TOP_LEVEL_ROLE_NAMES = {_ROOT, _TIMESTAMP, _SNAPSHOT, _TARGETS}
 
 # T is a Generic type constraint for Metadata.signed