chore: document Ruby 3.4 requirement in README

[devin-ai-integration]

Summary

Docs-only. The README's build instructions never state a Ruby requirement, but the example genuinely cannot install on older Rubies: both launchdarkly-openfeature-server-sdk (0.2.0) and openfeature-sdk (0.6.5) declare required_ruby_version >= 3.4. On Ruby 3.0 bundle install fails with:

Because launchdarkly-openfeature-server-sdk >= 0.2.0 depends on Ruby >= 3.4
  and Gemfile depends on launchdarkly-openfeature-server-sdk ~> 0.2,
  Ruby >= 3.4 is required.

This matches the ruby-version: 3.4 already used in CI. No dependency changes: every Gemfile constraint already resolves to the latest published version (launchdarkly-server-sdk 8.14.0, launchdarkly-openfeature-server-sdk 0.2.0 under ~> 0.2, openfeature-sdk 0.6.5, listen 3.10.0 under ~> 3.3) and no Gemfile.lock is committed, so there is nothing to bump.

Testing

Built and ran the example locally on Ruby 3.4.7 against a real LaunchDarkly environment:

$ LAUNCHDARKLY_SDK_KEY=*** bundle exec ruby main.rb
*** The sample-feature feature flag evaluates to false.

Also ran with RUBYOPT=-w: no deprecation warnings originate from main.rb; the noise is internal to third-party gems (http-cookie, openfeature-sdk circular require, LD SDK unused-variable warnings). No deprecated LD SDK or OpenFeature APIs are used — set_provider_and_wait and fetch_boolean_value are both current in openfeature-sdk 0.6.5.

Link to Devin session: https://app.devin.ai/sessions/577d0b569d8f4f01b835426cb5c7a202
Requested by: @jsonbailey

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[devin-ai-integration]

@cursor review