Prefer chart-rendered BBMs wait Job, keep inline fallback

What does this MR do?

Prefer chart-rendered batched background migrations wait Job, keep inline fallback

During a zero-downtime upgrade the operator waited for batched background migrations by synthesizing a Job in Go (deep-copying the migrations Job and overwriting its args with an inline poll loop). This moves that logic out of the operator: it now prefers the wait Job the chart renders, and only falls back to the inline derivation for charts that predate it.

• Selects the chart Job by the gitlab.com/batched-background-migrations-check label (partitioned out before the migrations Job, which shares app: migrations).
• Enables migrations.batchedBackgroundMigrationsCheck only while upgrading.
• Keeps the inline poll-loop derivation as a fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Implements #2223

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