awk: Flag -Ft as deprecated behavior

Upstream is poised to deprecate the -Ft wart in one true awk. None of
the other awks do this, and the gawk maintainer says that he's had no
requests for it in gawk in 30 years maintaining it. github can find a
few instances of it in the wild. As such, warn that it's deprecated and
will go away in the future.

MFC After: 3 days
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