Remove deprecated (and soon to be removed) --metrics-port usage from flagd-proxy startup.

Remove [these instances](https://github.com/search?q=repo%3Aopen-feature%2Fopen-feature-operator+metrics-port&type=code) of deprecated `--metrics-port` and use `--management-port` instead.

See related flagd change.

:warning: this MUST be done before consuming the next release of flagd-proxy