Remove old cache configuration

Description

In 11.3 !968 (merged) we introduced GCS as a new cache provider for GitLab Runner. We now split the s3 specific configuration in [runners.cache.s3] as specified in here, we did this while still supporting the old configuration.

Proposal

Remove support for the old configuration and cli flags.

Links to related issues and merge requests / references

Main deprecation issue #3473 (closed) MR introduced the new configuration !968 (merged)