This fixes one of the issues noted at <https://issues.guix.gnu.org/50425#4>.
* guix/import/minetest.scm
(release-version): New procedure.
(%minetest->guix-package): Call new procedure instead of release-title.
* tests/minetest.scm
(make-package-sexp): Allow overriding the version number.
(make-releases-json): Allow overriding the release title.
("conventional version number")
("v-prefixed version number")
("dates as version number"): New tests.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>