CI: Use OpenBSD 7.7 instead of 7.6 (squid-cache#2275)

Fix staging checks by targeting the current version of OpenBSD.

    https://cdn.openbsd.org/pub/OpenBSD/7.6/packages/amd64/: no such dir

Author: kinkie

.github/workflows/slow.yaml

@@ -198,12 +198,12 @@ jobs:
         uses: vmactions/openbsd-vm@v1
         with:
           usesh: true
-          release: "7.6"
+          release: "7.7"
           prepare: |
             pkg_add \
-              autoconf-2.72p0 \
+              autoconf%2.72 \
               autoconf-archive \
-              automake-1.16.5 \
+              automake%1.16 \
               bash \
               coreutils \
               cppunit \