Pick up config.cron.default_timezone from Rails config

[st0012]

Since Rails apps should have their timezone set, I think we can pick up config.cron.default_timezone from it.

[codecov]

Codecov Report

Merging #2213 (a2f945e) into master (babdd55) will increase coverage by 30.89%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2213       +/-   ##
===========================================
+ Coverage   66.48%   97.38%   +30.89%     
===========================================
  Files         100      102        +2     
  Lines        3742     3818       +76     
===========================================
+ Hits         2488     3718     +1230     
+ Misses       1254      100     -1154     

Components	Coverage Δ
sentry-ruby	98.07% <ø> (+42.16%)	⬆️
sentry-rails	95.01% <100.00%> (+0.03%)	⬆️
sentry-sidekiq	94.70% <ø> (ø)
sentry-resque	92.06% <ø> (ø)
sentry-delayed_job	94.44% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-rails/lib/sentry/rails/railtie.rb	98.75% <100.00%> (+0.06%)	⬆️

... and 55 files with indirect coverage changes