fix: Remove blank and template pages in sitemap.xml

Step 1: What is changing in this MR?

Production	Review app	Entry (If Apply)
https://about.gitlab.com/gitlab-diversity-inclusion-belonging	https://3630-blank-and-template-pages-in-sitemap-xml.about.gitlab-review.app/gitlab-diversity-inclusion-belonging/	Entry (Already published)
https://about.gitlab.com/devsecops-world-tour	https://3630-blank-and-template-pages-in-sitemap-xml.about.gitlab-review.app/devsecops-world-tour	Entry (Should be published after merging this MR)
https://about.gitlab.com/events/devsecops-world-tour-jp	https://3630-blank-and-template-pages-in-sitemap-xml.about.gitlab-review.app/events/devsecops-world-tour-jp	Entry (Already unpublished, if approved, entry will be deleted)
https://about.gitlab.com/events/standard-event-template	https://3630-blank-and-template-pages-in-sitemap-xml.about.gitlab-review.app/events/standard-event-template	Entry (Already unpublished, if approved, entry will be deleted)

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

Closes #3630 (closed)