Rails 5: fix mysql milliseconds problem in prometheus alert event spec

What does this MR do?

Fix rails 5 mysql milliseconds problems in new prometheus alert event spec. Related to gitlab-ce#48430. Fixes ee/spec/models/prometheus_alert_event_spec.rb:{56,82} see https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/105333996.

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• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
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• Conforms to the database guides
• EE specific content should be in the top level /ee folder
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