Add support for configuring multiple regions gitlab-tech-demo-series in CMS

Step 1: What is changing in this MR?

• Add support for multiple regions to a single event -> Contentful entry
• Legacy configuration for single region still works -> Contentful entry

Before:

{
 "region": "AMER"
}

After:

{
 "regions": [
     "AMER",
     "EMEA"
  ]
}

Production	Review app
https://about.gitlab.com/	https://3711-marketing-campaigns-request-technical-demo-series-page-add.about.gitlab-review.app/gitlab-technical-demo-series/

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Build Variables:

• Use Contentful Preview API

Related to #3711 (closed)