Adding types to elm_json_updater.rb

[robaiken]

What are you trying to accomplish?

Add Sorbet type checking for elm_json_updater.rb

How will you know you've accomplished your goal?

All CI checks should pass and there shouldn't be any new errors in Sentry.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[markhallen]