Update keela to 0.5.0

What does this MR do and why?

Bumps the keela dead-code detection gem from 0.4.1 to 0.5.0, and regenerates the baseline to pick up 0.5.0's deduplication fix.

Two commits:

1. Update keela to 0.5.0 — Gemfile (~> 0.4.1 → ~> 0.5.0) plus Gemfile.lock and Gemfile.next.lock, regenerated with bundle lock --update=keela. Only keela changed; its transitive dependencies are unaffected.
2. Regenerate keela baseline for 0.5.0 deduplication — keela 0.5.0 no longer lists an unused entry more than once per file when a single definition is extracted from multiple lines (a method delegated twice, or a constant declared and referenced). See https://github.com/kerrizor/keela/pull/72. Regenerating scripts/lint/keela_baseline.yml removes four such duplicate entries. No new unused code was surfaced — this is purely the dedup fix taking effect.

Notable 0.5.0 changes

• Minimum required Ruby is now 3.3.0 (was 3.1.0). Satisfied: this project runs Ruby 3.3.x.
• Faster usage detection (regex fast-path improvements).
• Fix: usage matches can no longer span a file boundary.
• New opt-in partials strategy (not in the default set, so it does not affect the default baseline).

Full changelog: https://github.com/kerrizor/keela/blob/main/CHANGELOG.md

How to set up and validate locally

bundle install
bundle exec keela --version   # => keela 0.5.0
bundle exec keela             # baseline is clean