Enable ClickHouse migrations by default

What does this MR do and why?

Enable ClickHouse migrations by default

It turns out that we run migration differently on .com. See more in Clickhouse - automatically run migrations durin... (#428126 - moved)

So this feature flag can't really be tested on .com

On the self-managed we migrate CH only if it's configured:

https://gitlab.com/gitlab-org/gitlab/blob/716a459d47bab3945e6a86c9236680f424fefa68/lib/tasks/gitlab/click_house/migration.rake#L11

Otherwise, we just print a message.

But let's keep the feature flag for a few milestones, just in case our experimental-customers would want to disable it.

(I'm probably being too paranoid here)

See more in Run ClickHouse migrations during gitlab:db:conf... (!138661 - merged)

Related issues:

• Clickhouse - automatically run migrations durin... (gitlab-org/charts/gitlab#5043 - closed)
• Clickhouse - automatically run migrations durin... (omnibus-gitlab#8266 - closed)

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