Clean up gem's RuboCop configuration

What does this MR do and why?

This MR Cleans up gem's RuboCop configuration:

• Disable RSpec/AvoidConditionalStatements for all gems
• Remove duplicates
• Remove unnecessary excludes
• Fix all RuboCop offenses related to Lint/RedundantCopDisableDirective

Contributes to #417019

How to set up and validate locally

$ for i in $(ls gems/*/.rubocop.yml); do (cd $(dirname $i) && pwd && bundle --quiet && bundle exec rubocop
 --ignore-parent-exclusion); done
/home/peter/devel/gitlab/gdk/gitlab/gems/activerecord-gitlab
Inspecting 8 files
........

8 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/click_house-client
Inspecting 11 files
...........

11 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/gitlab-rspec
Inspecting 9 files
.........

9 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/gitlab-schema-validation
Inspecting 66 files
..................................................................

66 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/gitlab-utils
Inspecting 12 files
............

12 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/ipynbdiff
Inspecting 15 files
...............

15 files inspected, no offenses detected
/home/peter/devel/gitlab/gdk/gitlab/gems/rspec_flaky
Inspecting 17 files
.................

17 files inspected, no offenses detected

📓 After !126163 (merged) merged --ignore-parent-exclusion is not longer needed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.