Fix API elasticsearch settings update

What does this MR do?

This MR fixes the exception when user tries to update elasticsearch_shards or elasticsearch_replicas via API.

This bug was introduced in !57567 (merged) where we changed the update logic for these settings.

PUT http://localhost:3000/api/v4/application/settings?elasticsearch_shards=16

NoMethodError (undefined method `to_h' for "16":String
Did you mean?  to_d
               to_r
               to_f
               to_i
               to_s
               to_c):
  /home/dgruzd/projects/gdk-ee/gitlab/ee/app/services/ee/application_settings/update_service.rb:54:in `update_elasticsearch_index_settings'
  /home/dgruzd/projects/gdk-ee/gitlab/ee/app/services/ee/application_settings/update_service.rb:29:in `execute'
  /home/dgruzd/projects/gdk-ee/gitlab/lib/api/settings.rb:227:in `block in <class:Settings>'

#329669 (closed)

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Does this MR meet the acceptance criteria?

Conformity

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• [-] Tested in all supported browsers
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Closes #329669 (closed)