runtime: reject runtime variables with the runtime guard naming if they're not known flags

We runtime guard behavioral changes (default true) but if folks override the default behavior they (previously) have no warning when the code paths are removed and their override becomes a no-op. As of https://github.com/envoyproxy/envoy/pull/19880 we will ENVOY_BUG for unknown flags which is better than nothing, but I propose we outright reject configs with unknown flags, in case folks aren't paying attention to their logs.