Cleanup CI images, add distro release and version

[bthomee]

• Replaces shell: bash in each step by setting the shell in defaults.
• Uses the concurrency group to stop building an image when a new commit is added.
• Removes setting the compiler.cppstd=20 option as that is now provided by the default Conan profile in the rippled repo.
• Removes the TODOs since they have been addressed.
• Removes unused CXX_VER.
• Adds the distro release and version to the Conan configuration that affects generating the package IDs.