Upgrade to latest CEL specification

[slott56]

CEL specification update:

• Compare CEL specification 0.5.0 (when this was released) with "current" CEL specification (0.24.0 as of this edit) to get the list of changes.
• Update all documentation, README, and all .feature files with very visible details on specific CEL specification used.
• Rewrite and expand test cases to cover "current" CEL specification (0.24.0).
• Prioritize resulting rework to bring this project into compliance. Create individual issues and a parent tracking issue for CEL compliance.

[slott56]

With PR #131, we now have tooling (and test cases) for the last CEL specification. (Thanks @hudlow and @stefanvanburen for doing so much work on this.)

This means we "simply" have a long list of @wip test cases.

The fourth of the sub-issues starts with triage of nearly 1200 new test cases into at least three categories: won't fix, hard-to-fix, and easy-to-fix. I'm going to open a discussion on @wip tests, to avoid cluttering this issue with a lot of digressions unrelated to the core four sub-issues.