Disallow creation of new issues for Trisquel versions < 6.0

Trisquel < 4.0 are unsupported, and 4.0 desktop is unsupported, while server support is supposed to end tomorrow, so the only Trisquel versions under which users should be allowed to file issues are 6.0 and 7.0. Right now, it is possible to select a version as far back as 2.0 in "Trisquel" and 4.0 in "Trisquel mini".