docs: update README to reflect Ruby 4.0 testing support

Update the Requirements section to indicate that we test up to Ruby 4.0
instead of Ruby 3.4, reflecting the current state of our test matrix.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: mjq

sentry-ruby/README.md

@@ -33,7 +33,7 @@ If you're using `sentry-raven`, we recommend you to migrate to this new SDK. You
 
 ## Requirements
 
-We test from Ruby 2.4 to Ruby 3.4 at the latest patchlevel/teeny version. We also support JRuby 9.0.
+We test from Ruby 2.4 to Ruby 4.0 at the latest patchlevel/teeny version. We also support JRuby 9.0.
 
 If you use self-hosted Sentry, please also make sure its version is above `20.6.0`.