Fix rendering of confirm_merge partial

[aaronskiba]

Changes proposed in this PR:

• This change simplifies the partial naming by taking advantage of Rails' naming conventions and resolves the following error:

ActionView::MissingTemplate - Missing partial super_admin/users/_confirm_merge.html.erb with {:locale=>[:"en-CA", :en, :"en-GB"], :formats=>[:js, :html, :text, :css, :ics, :csv, :vcf, :vtt, :png, :jpeg, :gif, :bmp, :tiff, :svg, :webp, :mpeg, :mp3, :ogg, :m4a, :webm, :mp4, :otf, :ttf, :woff, :woff2, :xml, :rss, :atom, :yaml, :multipart_form, :url_encoded_form, :json, :pdf, :zip, :gzip, :docx, :turbo_stream], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :jbuilder]}.
...
  app/controllers/super_admin/users_controller.rb:94:in `search'

• The prior 'super_admin/users/confirm_merge.html.erb' code works with releasev4.2, which uses Rails 6. However, it generates the following deprecation warning:

DEPRECATION WARNING: Rendering actions with '.' in the name is deprecated: super_admin/users/_confirm_merge.html.erb (called from search at /home/aaron/Documents/GitHub/roadmap_upstream/app/controllers/super_admin/users_controller.rb:94)

NOTE:

A pull request with these same changes was previously approved and merged into development (#3515).