Warn users about config.async's deprecation

[st0012]

Since we're going to drop config.async in 6.0 (as explained in #1522), I think we should start warning users about the change.

I did my best to make the message clear and compact, but it'd be great if we can have real professionals to polish it a bit 🙂

[codecov-commenter]

Codecov Report

Merging #1803 (f3b17fb) into master (7429e6c) will decrease coverage by 1.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1803      +/-   ##
==========================================
- Coverage   98.42%   96.55%   -1.88%     
==========================================
  Files         145      120      -25     
  Lines        8588     7308    -1280     
==========================================
- Hits         8453     7056    -1397     
- Misses        135      252     +117     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/configuration.rb	98.43% <100.00%> (+<0.01%)	⬆️
sentry-rails/lib/sentry/rails/railtie.rb	26.02% <0.00%> (-71.24%)	⬇️
sentry-rails/lib/sentry/rails/tracing.rb	34.21% <0.00%> (-65.79%)	⬇️
sentry-rails/lib/sentry/rails/active_job.rb	37.83% <0.00%> (-62.17%)	⬇️
...entry-rails/lib/sentry/rails/capture_exceptions.rb	36.00% <0.00%> (-60.00%)	⬇️
sentry-rails/lib/sentry/rails/backtrace_cleaner.rb	47.05% <0.00%> (-52.95%)	⬇️
.../lib/sentry/rails/rescued_exception_interceptor.rb	50.00% <0.00%> (-50.00%)	⬇️
...ntry/rails/tracing/action_controller_subscriber.rb	52.94% <0.00%> (-47.06%)	⬇️
...ls/lib/sentry/rails/tracing/abstract_subscriber.rb	37.03% <0.00%> (-37.04%)	⬇️
.../sentry/rails/tracing/active_storage_subscriber.rb	63.63% <0.00%> (-36.37%)	⬇️
... and 31 more

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