Track created_at on CI minutes records

What does this MR do and why?

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/331707

This MR adds created_at column for Ci::Minutes::NamespaceMonthlyUsage and Ci::Minutes::ProjectMonthlyUsage models.

This allows us to track exactly when the records for these 2 models are lazily created and compare the times with when we currently reset CI minutes on our legacy tracking: namespace_statistics.shared_runners_seconds_last_reset.

More context at https://gitlab.com/gitlab-org/gitlab/-/issues/331707#note_731715411

Database migrations

$ rails db:migrate
== 20211115145107 AddCreatedAtToNamespaceMonthlyUsages: migrating =============
-- transaction_open?()
   -> 0.0000s
-- add_column(:ci_namespace_monthly_usages, :created_at, :datetime_with_timezone)
   -> 0.0026s
== 20211115145107 AddCreatedAtToNamespaceMonthlyUsages: migrated (0.0486s) ====

== 20211115154103 AddCreatedAtToProjectMonthlyUsage: migrating ================
-- transaction_open?()
   -> 0.0000s
-- add_column(:ci_project_monthly_usages, :created_at, :datetime_with_timezone)
   -> 0.0016s
== 20211115154103 AddCreatedAtToProjectMonthlyUsage: migrated (0.0073s) =======


$ rails db:rollback STEP=2
== 20211115154103 AddCreatedAtToProjectMonthlyUsage: reverting ================
-- transaction_open?()
   -> 0.0000s
-- remove_column(:ci_project_monthly_usages, :created_at)
   -> 0.0029s
== 20211115154103 AddCreatedAtToProjectMonthlyUsage: reverted (0.0350s) =======

== 20211115145107 AddCreatedAtToNamespaceMonthlyUsages: reverting =============
-- transaction_open?()
   -> 0.0000s
-- remove_column(:ci_namespace_monthly_usages, :created_at)
   -> 0.0015s
== 20211115145107 AddCreatedAtToNamespaceMonthlyUsages: reverted (0.0056s) ====

MR acceptance checklist

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