Rename cancellable? to cancelable? in AutoCancelPolicy

What does this MR do and why?

Renames the private predicate cancellable? to cancelable? in Gitlab::Ci::RedundantPipelines::AutoCancelPolicy, and fixes the same double-l spelling in nearby comments and RSpec example names.

The codebase prefers the single-l US spelling. Counting app/ and lib/:

Pair	Count
cancelable / cancellable	209 / 6
canceling / cancelling	432 / 6
canceled / cancelled	1520 / 656

The rename also makes the predicate consistent with its public caller, cancelable_by_newer_pipeline?.

cancellation is left as-is. The double l is correct there in both US and UK English, so safe_cancellation: and similar are already right.

The method is private and the class has no callers yet, so this is a pure rename with no behaviour change.

app/services/ci/pipeline_creation/cancel_redundant_pipelines_service.rb is deliberately untouched. It holds a few double-l names, but three of them are structured log field names that external dashboards may query.

How to set up and validate locally

bundle exec rspec spec/lib/gitlab/ci/redundant_pipelines/auto_cancel_policy_spec.rb

27 examples, 0 failures. reek and rubocop are clean on both files.

References

• Review comment: !257237 (comment 3899631855)
• Discussion issue: #621543

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