Fine tune over limit email notification worker

What does this MR do and why?

Fine tune over limit email notification worker as current method times out for sql statements - see https://gitlab.com/gitlab-org/gitlab/-/issues/413547#note_1422454511

As per !118906 (comment 1420459287)

• Remove skip locked since we only use one worker with a 24hr buffer
• Use read replica for reads
• load the namespace_details in a separate query

Database

• select namespaces for the update - https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19300/commands/63702 (~530ms)
• update those namespaces selected - (ids 102-202 as a simulation of the output from the select) - https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19300/commands/63705 (~45ms)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #413547