Update CBMC starter kit

[angelonakos]

Description

This PR:

• changes the SPDX identifier in a single CBMC proof's Makefile
• introduces updates to the files related to the CBMC starter kit (this was accomplished by running cbmc-starter-kit-update).
• removes 2 .git submodules (this was accomplished by running cbmc-starter-kit-update --remove-starter-kit-submodule --remove-litani-submodule). symlinks to files present in the CBMC starter-kit submodule are no longer present Additionally, a user will have to install litani (instructions here) in order to run CBMC proofs locally. litani is present in the CI system that executes the CBMC proofs.
• updates the README to include a section on CBMC.

Checklist:

Changes pertain to CBMC proofs, therefore, I perceive the following 2 boxes as not applicable.

• [ ] I have tested my changes. No regression in existing tests.
• [ ] My code is formatted using Uncrustify.
• [x] I have read and applied the rules stated in CONTRIBUTING.md.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[angelonakos]

The latest force-push concerned solely the SPDX identifier change.