[Code quality] Set FlowCollector namespace field immutable

Having the ability to change namespace "on the fly" seems to have a disproportionate ratio of complexity / feature benefit : it brings quite a lot of complexity in operator code, for something likely hardly used and that would have a simple workaround if not implemented (removing / recreating the FlowCollector resource).

Making it immutable would simplify operator code a lot

To make it immutable with kubebuilder, cf https://kubernetes.io/blog/2022/09/29/enforce-immutability-using-cel/#immutablility-after-first-modification