Automatic upgrade of CBMC from 6.7.1 to 6.8.0 (model-checking#4448)

Upgrade CBMC to its latest release.

Co-authored-by: tautschnig <1144736+tautschnig@users.noreply.github.com>

Author: github-actions[bot]

kani-dependencies

@@ -1,5 +1,5 @@
 CBMC_MAJOR="6"
-CBMC_MINOR="7"
-CBMC_VERSION="6.7.1"
+CBMC_MINOR="8"
+CBMC_VERSION="6.8.0"
 
 KISSAT_VERSION="4.0.1"