Downgrade vala from 0.36.9 to 0.36.7 and bump PORTEPOCH. Vala versions
0.36.8 and 0.36.9 incorrectly generate vte-2.91.vapi when building
x11-toolkits/vte3, which breaks the build of x11/altyo.

PR: 225476
Reported by: antoine (portmgr)