[openstreetmap/openstreetmap-website] Remove json dependency (PR #6348)

[rkoeze]

### Description
Ruby has shipped with the default JSON gem since 1.9, so there's no need to require this in the Gemfile unless we want to pin a version (which I don't think we do). 

I don't think `multi_json` requires us to explicitly require the gem either. They mention in their docs [the JSON gem that ships with Ruby is supported](https://github.com/sferik/multi_json?tab=readme-ov-file#supported-json-engines). I also don't see anything concerning in the `multi_json` [adapter](https://github.com/sferik/multi_json/blob/main/lib/multi_json/adapters/json_gem.rb#L7).  

### How has this been tested?
The tests pass.

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6348

-- Commit Summary --

  * Remove json dependency

-- File Changes --

    M Gemfile (3)
    M Gemfile.lock (1)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6348.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6348.diff

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