Fix version of reverse_proxy_plug to 2.3.0

[taku1201]

Description

In Import Content section on hubs admin console, we input valid source url and click PREVIEW IMPORT button, then we met an issue that reticulum returns HTTP 500 error.
We investigated this issue and we found that version 2.3.0 of reverse_proxy_plug would not cause this problem.
As the current mix.exs configuration seems to allow installation for any minor versions from 2.1.x onwards, so we propose to pin it to 2.3.0.

reverse_proxy_plug >= 2.3.1

reverse_proxy_plug == 2.4.0

$ docker compose exec reticulum cat /ret/releases/1.0.0/ret.rel | grep reverse_proxy_plug
          {reverse_proxy_plug,"2.4.0",permanent},

reticulum returns 500 error.

reverse_proxy_plug == 2.3.1

$ docker compose exec reticulum cat /ret/releases/1.0.0/ret.rel | grep reverse_proxy_plug
          {reverse_proxy_plug,"2.3.1",permanent},

reticulum returns 500 error too.

Sentry Event: https://taku-ito.sentry.io/share/issue/25e12e1405ac415d9c81ab38fb34d17e/

reverse_proxy_plug <= 2.3.0

$ docker compose exec reticulum cat /ret/releases/1.0.0/ret.rel | grep reverse_proxy_plug
          {reverse_proxy_plug,"2.3.0",permanent},

reticulum returns proper result and the import was successful.

[taku1201]

This line should have been changed in ef3fde0, but it has not.
Therefore, we change this line to match the current situation.

[keianhzo]

LGTM Thanks!

[hrithikwins]

Much needed, thanks a lot