Consistent string freezing

[st0012]

Currently, some string literals are frozen with the frozen literal comment, some with freeze calls, and some are not frozen at all.

I think we can standardize it with RuboCop so it's easier to maintain.

I recommend reviewing by commits and I added a changelog for this too in case the newly frozen string causes some unexpected issues.

[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.66%. Comparing base (8dc3d2d) to head (2f5f18b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
sentry-ruby/spec/sentry/client_spec.rb	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2422   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files         218      218           
  Lines       14337    14339    +2     
=======================================
+ Hits        14145    14147    +2     
  Misses        192      192           

Components	Coverage Δ
sentry-ruby	99.00% <95.83%> (-0.01%)	⬇️
sentry-rails	97.34% <100.00%> (ø)
sentry-sidekiq	97.07% <100.00%> (ø)
sentry-resque	97.11% <ø> (+0.32%)	⬆️
sentry-delayed_job	98.92% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-delayed_job/lib/sentry-delayed_job.rb	73.33% <ø> (ø)
...elayed_job/lib/sentry/delayed_job/configuration.rb	100.00% <ø> (ø)
sentry-delayed_job/spec/sentry/delayed_job_spec.rb	100.00% <ø> (ø)
sentry-rails/app/jobs/sentry/send_event_job.rb	60.00% <ø> (ø)
sentry-rails/lib/generators/sentry_generator.rb	100.00% <ø> (ø)
sentry-rails/lib/sentry-rails.rb	100.00% <ø> (ø)
sentry-rails/lib/sentry/rails.rb	100.00% <ø> (ø)
sentry-rails/lib/sentry/rails/action_cable.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/active_job.rb	100.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/background_worker.rb	100.00% <ø> (ø)
... and 123 more

... and 1 file with indirect coverage changes