net-p2p/syncthing: migrate 1.3.4 to EGO_SUM
Done ahead of the mass migration planned by williamh in order to work
around the "mod flag may be set only once" error in go-module.eclass.
Revbump because some of the dependencies might have changed - in the
past if a vendored module offered a recent-enough release I would use
it, now the ebuild uses verbatim whatever tags have been specified in
the syncthing go.sum file.
Bug: https://bugs.gentoo.org/711640
Signed-off-by: Marek Szuba <marecki@gentoo.org>