Update all development / test gems

[miry]

There are plenty new versions of gems that are outdated:

• hiredis-client - Bump hiredis-client from 0.5.1 to 0.7.1 #376
• redis - Compatibility issues with Redis 5.X #385
• minitest - Bump minitest from 5.16.2 to 5.16.3 #375
• rubocop - Bump rubocop from 1.35.0 to 1.36.0 #384
• rubocop-minitest - Bump rubocop-minitest from 0.20.1 to 0.21.1 #380
• rubocop-shopify - Bump rubocop-shopify from 2.8.0 to 2.9.0 #378

[muz]

@miry I've updated the ticket description to link to respective Dependabot PRs for the testing deps.

In the case of rubocop-shopify, I've added a commit to get the code to conform with the new enforced linting style that gem bump brings.

[muz]

Regarding Redis, I've made a specific issue about something we've encountered with Redis 5.X here; #385