package/openjdk: fully switch to Github, commonalise version scheme

Commit 057e2702 (package/openjdk{, -bin}: bump latest to version
16.0.1+9) partially switched over to using the Github repository (which
is the new official publication channel for OpenJDK).

However, only the JDK16 was switched, because of concerns about a change
in the hash of Github-generated archives for the JDK11, due to a missing
Hg-related file on Github.

But as Arnout put it:
    There's a trivial workaround: drop OPENJDK_SOURCE = .... That way,
    the tarball name becomes openjdk-... instead of jdk-... and it's a
    different file.

There is indeed no good reason to force a non-default filename for the
archive, so we do drop it.

As a consequence, we can fully switch over to Github for openjdk, using
the new version scheme. Of course the hash changes, but it is a new
file, so that's OK.

The filename for the JDK16 changes, but the content does not change, so
the hash does not change.

For consistency, the version scheme is also applied to openjdk-bin. Even
though it was already using Github, using that new version scheme also
allows to commonalise the variables too. The archives are the exact
same: no change in filename or content, so no hash to fixup.

Reported-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
cc: Adam Duskett <aduskett@gmail.com>
Tested-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>