Remove deprecated ginkgo flag from CATs run

[ctlong]

What is this change about?

• Removes --slow-spec-threshold flag.
• Removes SLOW_SPEC_THRESHOLD param from task.
• Gets rid of annoying warning message at the end of CATs runs.
• Prepares us for the flag being ripped out at any point

Please provide contextual information.

Warning message from ginkgo when this flag is used:

You're using deprecated Ginkgo functionality:
=============================================
  --slow-spec-threshold is deprecated --slow-spec-threshold has been deprecated and will be removed in a future version of Ginkgo.  This feature has proved to be more noisy than useful.  You can use --poll-progress-after, instead, to get more actionable feedback about potentially slow specs and understand where they might be getting stuck.

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Remove slow spec threshold option from run-cats task, as it's deprecated by ginkgo.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

None.