Fix issues in deprecation doc

What does this MR do?

While reviewing a deprecation item, I noticed a doubled word, normally caught by Vale. We've turned off Vale on the deprecation doc due to the relative links and the style being more like release post items than product documentation, but this means some important issues get missed. The majority of the changes here include:

• Deleting the "Delete above this line" sections, which often contained GitLab's.
• Removing all instances of GitLab's everywhere else.
• Fixing spelling issues, like cancelled and typos.
• Fixing incorrect product names.
• Fixing all other vale issues that would normally be error-level (pipeline failing).

Running Vale on this file highlights a huge number of issues that are easily fixed. This MR fixes a lot, but it's just a drop in the bucket and we might want to consider finding a way to lint this doc, or review it periodically.

Related issues

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.