RuboCop: Increase amount of files in cache

What does this MR do and why?

Currently, we need to inspect more files that fit into RuboCop's cache:

$ bundle exec rubocop --list-target-files | wc -l
25188

Let's adjust RuboCop's configuration accordingly.

Refs gitlab-com/www-gitlab-com!90278 (comment 678551515)

Screenshots or screen recordings

n/a

How to set up and validate locally

n/a

MR acceptance checklist

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