abandon_stale.py: Adapt to removal of digest authentication
abandon_stale.py: Adapt to removal of digest authentication

HTTP Digest authentication was removed in Gerrit 2.14 and now only basic
authentication is supported.

Change the default authentication to basic, but still support digest
for backwards compatibility with previous Gerrit versions.

Change-Id: I7ccf9d2c506ed79f1209f995856c42c4936b4872

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