[ruby34] Fix frozen string warning

[tjallingvanderwal]

This change fixes the warning below, that appears when running Ruby 3.4.

.../flipper/lib/flipper/instrumentation/log_subscriber.rb:60:
warning: literal string will be frozen in the future
(run with --debug-frozen-string-literal for more information)

[tjallingvanderwal]

FYI I've tried to run the test suite locally, but did not succeed to get it running (and have limited time to try). So relying on the CI here.

$ docker-compose run --rm app bundle install

...


Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    Ruby

    flipper was resolved to 1.3.4, which depends on
      Ruby (>= 2.6)

    rails (~> 8.0) was resolved to 8.0.2, which depends on
      Ruby (>= 3.2.0)

[jnunemaker]

Fixed this over in #922 I think. Thanks for alerting!