Use the generic GetPointer instead of a typed GetPointer*

[pleshakov]

After generics were added to go, we added generic GetPointer https://github.com/nginxinc/nginx-kubernetes-gateway/blob/7662c2118927b12c7eacf60ca720fce087ac9e55/internal/helpers/helpers.go#L65 and started using it instead of typed GetPointer* functions. However, we didn't update the existing code.

Acceptance criteria:

• Use the generic GetPointer instead of a concrete GetPointer*

[bjee19]

Should we remove the now unused typed GetPointer* functions from the codebase?

[kate-osborn]

Should we remove the now unused typed GetPointer* functions from the codebase?

Yep!