editors/texstudio: update to 2.12.8

• USE_GITHUB since this release is only in GH and not in SF
• Add LICENSE according to project's site (no license file is provided)
• Use DISTVERSION instead of PORTVERSION
• Flavorize port and make qt5 default (as in the previous version)

PR: 227097
Reported by: lebarondemerde@privacychain.ch
Approved by: tcberner (mentor), portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D15023