Moving toward v3

[2bndy5]

This is a checklist of tasks that should be implemented in v3. Work for this shall live in the v3 branch until ready for deployment.

• switch to cpp-linter-rs project. We could use a standalone binary instead of the python binding, but dependabot would not know what version is used. Alternatively, we could use the node.js binding, but packaging that in cpp-linter-action needs further investigation (because of native distributions on npmjs.org).
• switch clang-tools installer to non-python dependent project. I have not started development on this (yet). The intention here is to remove the dependence on a python runtime.
• Write a migration guide for v2 -> v3 and v1 -> v3. This should include changes to inputs only. Users can find information about outputs in our regular documentation because there are no breaking changes in the outputs.

Backward compatibility

We can retain v2 in a non-default branch (named v2). This would allow current workflows to keep working with

- uses: cpp-linter/cpp-linter-action@v2

Dependabot updates will not update non-default branches, but that's ok. The cpp-linter-rs project will begin uploading releases to pypi, so the cpp-linter version on v2 branch should be pinned to the last pure-python release.