Update database_snowplow_collector_hostname application setting on SaaS

What does this MR do and why?

Related to #408644 (closed).

The setting was added in !127650 (merged).

This updates the database_snowplow_collector_hostname application setting on SaaS using a migration.

After setting the hostname, a hardcoded constant with it is going to be removed from DatabaseEventsSnowplow.

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