Add gated batched background migrations wait Job to migrations chart

What does this MR do?

Add gated batched background migrations wait Job to migrations chart

Adds a wait Job to the migrations subchart that holds a zero-downtime upgrade until the current version's batched background migrations finish, so the operator no longer has to build that Job itself.

• New Job gated on migrations.batchedBackgroundMigrationsCheck.enabled (default off), labelled gitlab.com/batched-background-migrations-check and running /scripts/wait-for-batched-background-migrations.
• Extracts the migrations Job's shared init/volume/volumeMount fragments into helpers so the two Jobs share config without duplication (rendered output of the existing migrations Job is unchanged).

Changelog: added

Implements gitlab-org/cloud-native/gitlab-operator#2223

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