Resolve "Debug why the code execution time is much longer than before"

What does this MR do and why?

Reset the spec timeout to 300ms as https://jihulab.com/gitlab-cn/gitlab/-/blame/869e2fd86ec139c8239896db74642d0e68d89f42/jh/spec/spec_helper.rb?page=1#L30

The feature flag use_metric_definitions_for_events_list has been removed on upstream https://gitlab.com/gitlab-org/gitlab/-/merge_requests/127586, and setting the timeout value less than 300 is fine according to the result on my local by testing ee/spec/lib/ee/gitlab/usage/service_ping_report_spec.rb.

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Closes #3273 (closed)