Update CBMC starter kit

[angelonakos]

Issue #, if available:

Description of changes:

• changes the SPDX identifier in CBMC proof Makefile from MIT to MIT-0
• 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.

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[angelonakos]

The latest commit concerned solely the SPDX identifier change.