Organize SLA details under Support Service Levels heading

• slack thread
• actionable next steps

Why is this change being made?

I know in EULA and ToS we link directly to https://about.gitlab.com/support/#gitlab-support-service-levels heading, but details on preferred region support hours are down the page a bit https://about.gitlab.com/support/#effect-on-support-hours-if-a-preferred-region-for-support-is-chosen

This MR:

1. Moves all SLA information as subheadings under GitLab Support Service Levels heading
2. removes mention of "All Regions" as default if preferred region not specified, preferred region is a required field

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in [#mr-buddies][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

@lyle @jcolyer @JamesRLopes @jrreid