nixos/grafana: fix systemd unit
nixos/grafana: fix systemd unit

Remove MemoryDenyWriteExecute hardening as it breaks image rendering
plugin. Add CAP_NET_BIND_SERVICE to bind to low ports when needed.
Remove PrivateUsers and ProcSubset as upstream choose to remove it.

Upstream changes: <https://github.com/grafana/grafana/pull/40219>,
<https://github.com/grafana/grafana/pull/40178>,
<https://github.com/grafana/grafana/pull/40339> and
<https://github.com/grafana/grafana/pull/40815>.

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