Logically re-apply 64c70a8c4c ("doc: point out that nixos-21.05 has gnuradio
3.9"), because it was lost in the conversion from docbook to markdown, in
commit 32c2dd304d ("docs: nixos release notes to CommonMark (2105)").
(Apparently we have both .md and .xml release notes now, and CI fails
unless they have the same content (after .md processing), so update the
.xml file to match...)
ede1785d11