relnotes: copy "next" to "5.4" and clear "next"
This is the same thing as we've done for the previous few releases.
Differential Revision: https://phab.mercurial-scm.org/D8546
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}