Update .github build scripts- cleanup, use Ubuntu-20.04, use Node-20

[hans-schmidt]

The Ravencoin repository tests each new Pull Request submission by attempting to build binaries with the new PR. But for the past year, it has interpreted all PRs as bad because the code for the build system is broken. Specifically, it uses Ubuntu-18.04 which is no longer supported by Github (fails) and it uses Node-16 which is deprecated (generates errors).

This PR updates Ravencoin's build system to use Ubuntu-20.04 and Node-20. It also removes code related to AWS cache storage which Overstock used years ago, and it does some other code clean-up.

This PR is for the master branch. Another PR has been submitted for the develop branch. No changes are made to the actual Ravencoin source code outside of the build system.

Similar build system changes have already been implemented and tested on the Evrmore blockchain codebase.