[openstreetmap/openstreetmap-website] Remove `multi_json` library (PR #6349)

[rkoeze]

### Description
Removes the `multi_json` gem. `multi_json` was originally [added as a dependency for the omniauth-windowslive plugin](https://github.com/openstreetmap/openstreetmap-website/commit/5a28ff2901fd0387805240732a31194d198d1071). However, that plugin was replaced with `omniauth-microsoft_graph` in https://github.com/openstreetmap/openstreetmap-website/pull/4169. Since `omniauth-microsoft_graph` does not need `multi_json` we can remove it from the Gemfile.

### How has this been tested?
The tests pass and I was able to run the app locally. I've also looked through the other omniauth plugins and confirmed they do not rely on `multi_json` gem either:

- [omniauth](https://github.com/search?q=repo%3Aomniauth%2Fomniauth+MultiJson&type=code)
- [omniauth-apple](https://github.com/search?q=repo%3Anhosoya%2Fomniauth-apple%20MultiJson&type=code)
- [omniauth-facebook](https://github.com/search?q=repo%3Asimi%2Fomniauth-facebook%20MultiJson&type=code)
- [omniauth-github](https://github.com/search?q=repo%3Aomniauth%2Fomniauth-github%20MultiJson&type=code)
- [omniauth-google-oauth2](https://github.com/search?q=repo%3Azquestz%2Fomniauth-google-oauth2%20MultiJson&type=code)
- [omniauth-mediawiki](https://github.com/search?q=repo%3Atimwaters%2Fomniauth-mediawiki%20MultiJson&type=code)
- [omniauth-microsoft_graph](https://github.com/search?q=repo%3Asynth%2Fomniauth-microsoft_graph%20multi_json&type=code)
- [omniauth-rails_csrf_protection](https://github.com/search?q=repo%3Acookpad%2Fomniauth-rails_csrf_protection%20MultiJson&type=code)
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6349

-- Commit Summary --

  * Remove multi_json library

-- File Changes --

    M Gemfile (2)
    M Gemfile.lock (2)
    M config/initializers/omniauth.rb (2)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6349.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6349.diff

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