Migrate `sentry-delayed_job` to AJ-first integration

Part of getsentry/sentry-ruby#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`)