Skip CI finished builds backfill when ClickHouse is not configured

Narendranrequested to merge
narendran/skip-backfill-ch-without-clickhouse into master

What does this MR do and why?

Skips the BackfillCiFinishedBuildsToClickHouse batched background migration on instances where ClickHouse is not configured.

Problem: The backfill migration (!222415 (merged)) creates sync events in p_ci_finished_build_ch_sync_events for the last 180 days of CI finished builds. On self-hosted instances without ClickHouse configured, these sync events are created but never processed — the CiFinishedBuildsSyncWorker already guards against this via CiFinishedBuildsSyncService.enabled? which checks Gitlab::ClickHouse.configured?. Unlike the real-time sync (which preserves only 30 days and is lightweight), this backfill upserts 180 days of records unnecessarily.

Solution: Add a Gitlab::ClickHouse.configured? check at the start of the perform method in the EE migration class. This follows the same pattern used by the downstream CiFinishedBuildsSyncService and ensures the backfill is a no-op on instances without ClickHouse.

Since this is a runtime class change (not persisted in the DB), no re-queue of the BBM is needed — the change takes effect on the next batch execution.

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