Update CBMC starter kit

[angelonakos]

This PR:

• changes the SPDX identifier in CBMC proof Makefile from Apache-2.0 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.

[angelonakos]

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