Upgrade Sidekiq gem independently of redis-rb

Sidekiq 7 drops the redis gem dependency in favour of redis-client which is a a lower-level client that the redis gem wraps over in v5.

We can reduce the risk of upgrade by upgrading the Sidekiq gem separately from the redis gem.

This would require us to do the following in a single MR:

• fix api breakages introduced by sidekiq 7
• adopt the middleware-based observability hooks which redis-client provides and provide redis-client compatible configuration