Update the deprecation policy to indicate that zuul-jobs is no
longer tested with EOL platforms. Also explicitly switch the minimum
Python 3 documented to 3.6, and add a note to the tox-py34 and
tox-py35 jobs mentioning that they're no longer directly tested.
Move those jobs to the deprecated jobs list as well, to help
reinforce the point that their continued use is not recommended.
Change-Id: I2edbf8ea010caf7a7641e0d88f360965fc0b96ab