fix: Allow JSON gem v3

[luizkowalski]

BEGIN_COMMIT_OVERRIDE
feat: Expand version support for JSON gem v3
END_COMMIT_OVERRIDE

Summary

Recently, json has released v3. This PR relaxes the json gem version constraints:

Could not find compatible versions

Because launchdarkly-server-sdk >= 6.2.4 depends on json ~> 2.3
  and Gemfile depends on launchdarkly-server-sdk ~> 8.15,
  json ~> 2.3 is required.
So, because Gemfile depends on json ~> 3,
  version solving has failed.

Applications can install launchdarkly-server-sdk with JSON 3, while JSON 2.3 and later remain supported

The SDK doesn't use any of the deleted methods

Tracked Internally: SDK-3115

[jsonbailey]

@luizkowalski Thank you for the PR. This is now available in v8.18.0