docs: document git/python/Ubuntu/Debian release schedules
docs: document git/python/Ubuntu/Debian release schedules

Going purely on upstream package release cycles doesn't tell the whole
story: a lot of people run LTS distros which will have older versions
of software we want to support.

Build out a table for us to quickly reference when making decisions as
to what versions of git/python we want to support, and when we can drop
them.  This will also help to refer users to as why we made a specific
decision that might be affecting them.

Change-Id: I7aea24bbefd50e358aeacf11e8c15a346c8fb8a9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254572
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>
Reviewed-by: David Pursehouse <e7b5c6a4a3dbaca972851f8c075e19915752593f@collab.net>

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