Require Ruby 3.1 or newer

[drinkbeer]

Summary

• raise the minimum supported Ruby version from 2.7 to 3.1
• update the documented Ruby requirement
• refresh the lockfile so brotli_splice remains a direct development dependency rather than appearing under response_bank's runtime dependencies

Why

Dependabot resolves updates against the lowest Ruby version allowed by a gem's required_ruby_version. It currently selects Ruby 2.7.8, which cannot resolve brotli_splice 0.1.1 because that gem requires Ruby 3.1 or newer.

Ruby 3.1 also matches the minimum requirement of Rails 7.2, the oldest Rails version in the current CI matrix.

Testing

• bundle exec rake test — 62 runs, 332 assertions, 0 failures
• built the gem and verified required_ruby_version is >= 3.1.0
• GitHub Actions