Set default app dirs pattern

[st0012]

Instead of falling back to the default app dirs pattern when config.app_dirs_pattern is nil, we can simply set it to APP_DIRS_PATTERN by default.

This means we don't need to always check if config.app_dirs_pattern is set and can simply use it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (4db5f74) to head (abc80d8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2390      +/-   ##
==========================================
- Coverage   98.69%   98.68%   -0.01%     
==========================================
  Files         210      210              
  Lines       13923    13923              
==========================================
- Hits        13741    13740       -1     
- Misses        182      183       +1     

Components	Coverage Δ
sentry-ruby	99.06% <100.00%> (ø)
sentry-rails	97.34% <ø> (ø)
sentry-sidekiq	97.07% <ø> (ø)
sentry-resque	96.79% <ø> (-0.33%)	⬇️
sentry-delayed_job	98.92% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/backtrace.rb	96.66% <100.00%> (-0.06%)	⬇️
sentry-ruby/lib/sentry/configuration.rb	98.83% <100.00%> (+<0.01%)	⬆️
sentry-ruby/lib/sentry/profiler.rb	99.11% <100.00%> (ø)
sentry-ruby/spec/sentry/backtrace/lines_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/transport_spec.rb	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes