add target openbsd-7.8

Author: leleliu008

.github/workflows/testing-on-openbsd.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        openbsd-version: ['7.7', '7.6', '7.5', '7.4', '7.3', '7.2']
+        openbsd-version: ['7.8', '7.7', '7.6', '7.5', '7.4', '7.3', '7.2']
 
     steps:
       - uses: actions/checkout@v5