Decrease the default number of background worker threads by half

[st0012]

See #2297 for the discussion.

Closes #2297

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.72%. Comparing base (09fa602) to head (e6c6197).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2305   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files         203      203           
  Lines       13274    13282    +8     
=======================================
+ Hits        13105    13113    +8     
  Misses        169      169           

Components	Coverage Δ
sentry-ruby	99.11% <100.00%> (+<0.01%)	⬆️
sentry-rails	97.37% <ø> (ø)
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-ruby/lib/sentry/configuration.rb	98.80% <100.00%> (ø)
sentry-ruby/spec/sentry/background_worker_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/configuration_spec.rb	100.00% <100.00%> (ø)