relnotes: copy contents of "next" to "5.8" and clear "next"
The same procedure as every year^Wcycle.
Unlike previous cycles, I haven't actually recorded the copy because
we don't want to propagate further changes in "next" (from the stable
branch) to "5.8".
Differential Revision: https://phab.mercurial-scm.org/D10500
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}