GitHub Actions: adding openbsd-7.6, removing old versions of openbsd

Author: leleliu008

.github/workflows/cross-compile-for-openbsd-on-ubuntu.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        openbsd-release-version: ['7.5', '7.4', '7.3']
+        openbsd-release-version: ['7.6']
         # https://cdn.openbsd.org/pub/OpenBSD/
 
     runs-on: ubuntu-latest