Upgrade sidekiq-cron to v1.8.0

What does this MR do and why?

Upgrade sidekiq-cron so we don't need our custom monkey-patch

• https://my.diffend.io/gems/connection_pool/2.2.5/2.3.0

2.3.0
------

- Minimum Ruby version is now 2.5.0
- Add pool size to TimeoutError message

• https://my.diffend.io/gems/sidekiq-cron/1.4.0/1.8.0

## 1.8.0

- Fix deprecation warnings with redis-rb v4.8.0 (https://github.com/ondrejbartas/sidekiq-cron/pull/356)
- Fix poller affecting Sidekiq scheduled set poller (https://github.com/ondrejbartas/sidekiq-cron/pull/359)
- Fix default polling interval (https://github.com/ondrejbartas/sidekiq-cron/pull/362)
- Add italian locale (https://github.com/ondrejbartas/sidekiq-cron/pull/367)
- Allow disabling of cron polling (https://github.com/ondrejbartas/sidekiq-cron/pull/368)

## 1.7.0

- Enable to use cron notation in natural language (ie `every 30 minutes`) (https://github.com/ondrejbartas/sidekiq-cron/pull/312)
- Fix `date_as_argument` feature to add timestamp argument at every cron job execution (https://github.com/ondrejbartas/sidekiq-cron/pull/329)
- Introduce `Sidekiq::Options` to centralize reading/writing options from different Sidekiq versions (https://github.com/ondrejbartas/sidekiq-cron/pull/341)
- Make auto schedule loading compatible with Array format (https://github.com/ondrejbartas/sidekiq-cron/pull/345)

## 1.6.0

- Adds support for auto-loading the `config/schedule.yml` file (https://github.com/ondrejbartas/sidekiq-cron/pull/337)
- Fix `Sidekiq.options` deprecation warning (https://github.com/ondrejbartas/sidekiq-cron/pull/338)

## 1.5.1

-  Fixes an issue that prevented the gem to work in previous Sidekiq versions (https://github.com/ondrejbartas/sidekiq-cron/pull/335)

## 1.5.0

- Integrate Sidekiq v6.5 breaking changes (https://github.com/ondrejbartas/sidekiq-cron/pull/331)
- Add portuguese translations (https://github.com/ondrejbartas/sidekiq-cron/pull/332)

MR acceptance checklist

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