fix: add explicit dependency on logger for Ruby 4

[mudge]

Description

Ruby 4.0 moved "logger" from being a default gem to a bundled one (see https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/#stdlib-updates and https://stdgems.org/logger/) so it now needs to be added as an explicit dependency.

Without this, it will raise the following warning:

warning: logger used to be loaded from the standard library, but is not part of the default gems since Ruby 4.0.0.
You can add logger to your Gemfile or gemspec to fix this error.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (7d10c02) to head (3cf811b).
⚠️ Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2837      +/-   ##
==========================================
+ Coverage   90.13%   90.27%   +0.14%     
==========================================
  Files         132      138       +6     
  Lines        5301     5421     +120     
==========================================
+ Hits         4778     4894     +116     
- Misses        523      527       +4     

Components	Coverage Δ
sentry-ruby	97.26% <98.78%> (-0.03%)	⬇️
sentry-rails	55.50% <ø> (ø)
sentry-sidekiq	95.05% <ø> (ø)
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	99.32% <100.00%> (+0.01%)	⬆️
see 1 file with indirect coverage changes

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