Migrate `sentry-delayed_job` to AJ-first integration

[solnic]

Part of #2932.

Smallest existing gem touched by this rework. Delayed::Job itself is not AJ-based, so the gem's own DJ plugin path is unchanged — this phase only covers the case where users run AJ on top of DJ.

Tasks

• 2.1 Unregister from skippable_job_adapters (per Phase 1.6 decision). Strip the Railtie block at sentry-delayed_job.rb#L17-L23 if option (a) won, or keep an empty default if (b).
• 2.2 Add a thin spec file under the sentry-delayed_job test suite that configures the AJ adapter as :delayed_job and calls it_behaves_like "a Sentry-instrumented ActiveJob backend". Requires pulling the shared-examples files from sentry-rails's test support path via the Gemfile dev dependency graph.
• 2.3 Reposition README + gemspec as maintenance-mode: point users at sentry-rails (AJ) for AJ-based jobs; keep the DJ-plugin path for raw Delayed::Job.enqueue(...) users. Bug fixes only, no new features.

Depends on

Phase 1 (ActiveJob core integration in sentry-rails)

[linear-code]

RUBY-173