Use a Redis cache for Renovate

Renovate is currently using a disk cache. This cache is loaded and persisted by GitLab-CI on each request and is very large.

Additionally, multiple pipelines are likely to overwrite values when persisting them as there is no concurrency control.

Finally, different repositories each have their own cache. This could be made more efficient by using a shared cache.

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Proposal: run a Redis cache for Renovate

Renovate supports the use of Redis for caching: https://docs.renovatebot.com/self-hosted-configuration/#redisurl

Run a Redis cache, possibly using GCP's MemoryStore and use that as the cache.

This could be configured as a Group CI/CD variable, RENOVATE_REDIS_URL which would be stored at the gitlab.com/gitlab-com level, allowing all repositories under this namespace to use the cache.