Drop `async` configuration

[st0012]

Closes #1522

[github-actions[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

---

"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.68%. Comparing base (07aaf5e) to head (9df626a).

Additional details and impacted files

@@             Coverage Diff             @@
##           6.0-dev    #1894      +/-   ##
===========================================
- Coverage    98.71%   98.68%   -0.03%     
===========================================
  Files          202      201       -1     
  Lines        13294    13092     -202     
===========================================
- Hits         13123    12920     -203     
- Misses         171      172       +1     

Components	Coverage Δ
sentry-ruby	99.07% <100.00%> (-0.03%)	:arrow_down:
sentry-rails	97.26% <100.00%> (-0.12%)	:arrow_down:
sentry-sidekiq	96.98% <ø> (ø)
sentry-resque	96.76% <ø> (ø)
sentry-delayed_job	98.91% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-rails/lib/sentry/rails/active_job.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/rails/activejob_spec.rb	99.42% <ø> (-0.07%)	:arrow_down:
sentry-ruby/lib/sentry/background_worker.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/client.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/configuration.rb	98.77% <ø> (-0.03%)	:arrow_down:
sentry-ruby/lib/sentry/transport.rb	99.03% <100.00%> (-0.01%)	:arrow_down:
sentry-ruby/spec/sentry/background_worker_spec.rb	100.00% <ø> (ø)
...ntry-ruby/spec/sentry/client/event_sending_spec.rb	99.50% <100.00%> (-0.13%)	:arrow_down:
sentry-ruby/spec/sentry/configuration_spec.rb	100.00% <ø> (ø)
...try/transport/http_transport_rate_limiting_spec.rb	100.00% <100.00%> (ø)
... and 3 more

... and 1 file with indirect coverage changes

[getsantry[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

---

"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀