Fix deprecation warning when rendering "xml.atom"

What does this MR do and why?

This fixes the following deprecation warning:

DEPRECATION WARNING: Rendering actions with '.' in the name is deprecated: layouts/xml.atom (called from render at /builds/gitlab-org/gitlab/app/controllers/application_controller.rb:133)

Contributes to #342492 (closed)

How to verify locally

Run related specs locally and observe the lack of warning Rendering actions with '.' in the name is deprecated: layouts/xml.atom [...].

files=$(tff -f tests.yml $(git diff --name-only HEAD~))

echo $files
spec/controllers/dashboard/projects_controller_spec.rb spec/controllers/groups_controller_spec.rb spec/controllers/projects/commits_controller_spec.rb spec/controllers/projects/issues_controller_spec.rb spec/controllers/projects/merge_requests_controller_spec.rb spec/controllers/projects/tags_controller_spec.rb spec/controllers/projects_controller_spec.rb

bin/rspec $files

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