Minimum bison and flex versions

Per the meson thread, we also need to have a conversation about
what's the oldest bison and flex versions still worth supporting.
The ones that had been our reference points are no longer
represented in the buildfarm, and it seems not likely to be
worth resurrecting copies of those.

As fodder for discussion, here's a scraping of the currently-tested
versions.  (This counts only animals running configure, ie not MSVC.
Also, my query looked back a few months, so some recently-dead
animals are still here.)

			regards, tom lane