Backend: Sidekiq warns of unsafe JSON serialization

Problem

Noticed in !79291 (comment 828755219) by @jhyson:

I also wanted to note that I saw both these errors in log/sidekiq_client.log while I was testing this:

{"severity":"WARN","time":"2022-02-02T03:20:13.856Z","message":"Job arguments to MergeRequests::UpdateHeadPipelineWorker do not serialize to JSON safely. This will raise an error in\nSidekiq 7.0. See https://github.com/mperham/sidekiq/wiki/Best-Practices or raise an error today\nby calling `Sidekiq.strict_args!` during Sidekiq initialization.\n"}
{"severity":"WARN","time":"2022-02-02T03:09:02.237Z","message":"Job arguments to GitlabSubscriptions::NotifySeatsExceededWorker do not serialize to JSON safely. This will raise an error in\nSidekiq 7.0. See https://github.com/mperham/sidekiq/wiki/Best-Practices or raise an error today\nby calling `Sidekiq.strict_args!` during Sidekiq initialization.\n"}

Perhaps this is something we need to look into as well?

We need to check why Sidekiq is rasing this warning and what we need to do to solve that.

Proposal

It's sufficient to ensure that the hash data uses string keys.