Disallow unknown keys in feature-flags configmap

Currently, we don't perform any validation of the feature flags configmap. https://github.com/tektoncd/pipeline/pull/6935 adds validation for keys representing existing feature flags, but it's still possible to misspell a feature flag or add a flag that's no longer supported, thinking it has an effect (example: https://github.com/tektoncd/pipeline/pull/6937). We should validate that the configmap doesn't contain any unknown keys. We will probably need to fix https://github.com/tektoncd/pipeline/issues/4542 first, otherwise upgrades will cause problems if we remove feature flags.