Update OpenBSD to release 7.6

Packages for 7.4 no longer seem to be available from mirrors.

Author: tautschnig

.github/workflows/bsd.yaml

@@ -80,18 +80,18 @@ jobs:
         with:
           save-always: true
           path: .ccache
-          key: openbsd-7.4-gmake-${{ github.ref }}-${{ github.sha }}-PR
+          key: openbsd-7.6-gmake-${{ github.ref }}-${{ github.sha }}-PR
           restore-keys: |
-            openbsd-7.4-gmake-${{ github.ref }}
-            openbsd-7.4-gmake
+            openbsd-7.6-gmake-${{ github.ref }}
+            openbsd-7.6-gmake
       - name: ccache environment
         run: |
           echo "CCACHE_BASEDIR=$PWD" >> $GITHUB_ENV
           echo "CCACHE_DIR=$PWD/.ccache" >> $GITHUB_ENV
       - name: Build and Test
         uses: vmactions/openbsd-vm@v1
         with:
-          release: 7.4
+          release: 7.6
           run: |
             # apparently fail-on-error isn't the default here
             set -e -x