Add upgrade note for Sidekiq concurrency limiter job backlogs on Helm chart deployments

What does this MR do?

Adds an upgrade note to doc/update/versions/gitlab_18_changes.md documenting the known issue where GITLAB_SIDEKIQ_MAX_REPLICAS, set by default in the Helm chart starting in GitLab 18.9, activates the Sidekiq concurrency limiter on Self-Managed and Dedicated environments without KEDA-based autoscaling. This causes job backlogs, Redis memory growth, and delayed execution for workers including webhooks, audit events, and Geo replication.

The note covers:

• Affected versions (18.9.0 - 18.9.5, 18.10.0 - 18.10.3) and fixed patch levels
• Symptoms (job backlogs, Redis memory growth, delayed execution)
• Two mitigation options (per-worker feature flag, GITLAB_SIDEKIQ_MAX_REPLICAS=0)
• Geo secondary caveat (ConcurrencyLimit::ResumeWorker not running on secondaries)
• Links to the fix MR and backports

Related issues

• https://gitlab.com/gitlab-com/support/support-team-meta/-/issues/7591+
• Fix MR: Add feature flag to gate default Sidekiq concur... (!230713 - merged)
• Backport to 18.10.4: Backport of add feature flag to gate default S... (!231085 - merged)
• Backport to 18.9.6: Backport of add feature flag to gate default S... (!231297 - merged)
• Chart MR that introduced the change: Add GITLAB_SIDEKIQ_MAX_REPLICAS environment var... (gitlab-org/charts/gitlab!4348 - merged)
• Dedicated incident analysis: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/3175+

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