Move CBMC back to latest (#1026)

This reverts commit 9848a8c ("Pin CBMC
version to 5.81.0", PR #1022): CBMC 5.83.0 includes
diffblue/cbmc#7700, which fixes the problem
spotted by aws-c-common's tests.

Author: tautschnig

.github/workflows/proof_ci_resources/config.yaml

@@ -1,5 +1,5 @@
 cadical-tag: latest
-cbmc-version: "5.81.0"
+cbmc-version: latest
 cbmc-viewer-version: latest
 kissat-tag: latest
 litani-version: latest