RSpec: Refind group object to prune memoized values

What does this MR do and why?

In !104491 (merged) we started to memoize Group#has_project_with_service_desk_enabled? and this state was leaky between specs when using let_it_be(:group).

Let's use let_it_be_with_refind(:group) instead.

Closes #386135 (closed).

How to set up and validate locally

From #386135 (closed).

bin/rspec ./spec/models/group_spec.rb[1:64:1:1:1,1:64:1:2:1] --order defined --seed 39944 -e has_project_with_service_desk_enabled?

Finished in 22.16 seconds (files took 9 seconds to load
4 examples, 0 failures

On master

git checkout master

bin/rspec ./spec/models/group_spec.rb[1:64:1:1:1,1:64:1:2:1] --order defined --seed 39944 -e has_project_with_service_desk_enabled?

...

Finished in 11.06 seconds (files took 10.04 seconds to load)
4 examples, 2 failures

Failed examples:

rspec ./spec/models/group_spec.rb:2846 # Group has_project_with_service_desk_enabled? when service desk is enabled for subgroup project is expected to eq true
rspec ./spec/models/group_spec.rb:2831 # Group has_project_with_service_desk_enabled? when service desk is enabled for top level group is expected to eq true

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