rust: upgrade minimum `rayon` dependency
The 1.6.0 and 1.6.1 releases fixes a soundness issue and a performance issue
that are both important to us, since both correctness and performance are
paramount in the Rust extensions.
#require docutils gettext
Error: the current ro localization has some rst defects exposed by
moving pager to core. These two warnings about references are expected
until the localization is corrected.
$ $TESTDIR/check-gendoc ro
checking for parse errors
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.