Use Digital Ocean high-CPU servers for CE/EE testing

Right now our pipelines for testing CE/EE are growing past 1.5 hours. We should consider throwing more hardware and parallelization at the problem for now while trying to optimize the specs themselves.

We should consider using the Digital Ocean high-CPU servers.

@ayufan @tmaczukin @rymai Thoughts on doing this?

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Final plan:

• create a separated DigitalOcean account for the high-cpu Runner's usage: #2886 (closed)
• convert private-runners-manager-X for dev.gitlab.org to use c-2 on a new projects,
• if it works OK, convert private-runners-manager-X for gitlab.com to use c-2,
• if it works OK, convert gitlab-shared-runners-manager-X to use c-2,
• remove hc-shared-runners-manager-X since they are not needed at this point.