fix: add explicit dependency on logger for Ruby 4

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.

Author: mudge

CHANGELOG.md

@@ -1,3 +1,9 @@
+## Unreleased
+
+### Bug Fixes
+
+- Add explicit dependency on logger gem to fix Ruby 4.0 warning ([#2837](https://github.com/getsentry/sentry-ruby/pull/2837))
+
 ## 6.3.0
 
 ### Features

sentry-ruby/sentry-ruby.gemspec

@@ -30,4 +30,5 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
   spec.add_dependency "bigdecimal"
+  spec.add_dependency "logger"
 end