Fix warnings when running the tests locally

[stefannibrasil]

What github issue is this PR for, if any?

Resolves #6726

What changed, and why?

There are a bunch of warnings being generated when running the tests locally:

casa % bundle exec rspec
Run options: exclude {:ci_only=>true}

Randomized with seed 6314
......................./opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: already initialized constant Cssbundling::Tasks::LOCK_FILES
/opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: previous definition of LOCK_FILES was here
./opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: already initialized constant Cssbundling::Tasks::LOCK_FILES
/opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: previous definition of LOCK_FILES was here
./opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: already initialized constant Cssbundling::Tasks::LOCK_FILES
/opt/homebrew/lib/ruby/gems/3.3.0/gems/cssbundling-rails-1.4.3/lib/tasks/cssbundling/build.rake:24: warning: previous definition of LOCK_FILES was here

After upgrading bundler to the latest version, the warnings get resolved locally. In CI, because they run in parallel it seems, these warnings still get printed out. But this changes keeps local env cleaner and the project up to date wit the latest Bundler version.

Running the tests locally after this update:

casa % bundle exec rspec
Run options: exclude {:ci_only=>true}

Randomized with seed 48450

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