Adjust proof tooling to support CBMC v6 (aws#87)

*Description of changes:*

With CBMC v6, unwinding assertions are enabled by default, and object
bits no longer need to be set at compile time. Update various build
rules to use the latest template as provided with CBMC starter kit.

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Author: tautschnig

.github/workflows/ci.yml

@@ -141,7 +141,9 @@ jobs:
     steps:
       - name: Set up CBMC runner
         uses: FreeRTOS/CI-CD-Github-Actions/set_up_cbmc_runner@main
+        with:
+          cbmc_version: "6.3.1"
       - name: Run CBMC
         uses: FreeRTOS/CI-CD-Github-Actions/run_cbmc@main
         with:
-          proofs_dir: test/cbmc/proofs
+          proofs_dir: test/cbmc/proofs