Rename from networking to network

We have `wingpanel-indicator-network`, why do we have `switchboard-plug-networkING` too?

- [ ] The meson project name is still networking,
- [ ] the debian package is still called switchboard-plug-networking,
- [ ] translation files are called networking-plug,
- [ ] and it's still referred to as the Networking Plug in the README.md file.

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