relnotes: copy "next" to "5.2" and clear "next"
This is the same thing as we did for 5.1 in cba59b338976 (relnotes:
copy "next" to "5.1" and clear "next", 2019-08-01).
Differential Revision: https://phab.mercurial-scm.org/D7231
#require docutils gettext $ $TESTDIR/check-gendoc ru checking for parse errors