Update vale warnings

What does this MR do?

Some minor cleanup of vale warnings, related to the conversations at !102622 (comment 1157073734) and !102622 (comment 1160605937). Simplifies the structure of the wordy rule, but also bumps a couple options up to warning:

• Changes the dropdown list rule from suggestion to warning.
• Moves the docs, e-mail and within rules from the suggestions rule to the warnings rule, as these are all trivial to fix.
• Fixes the overlap between the GFM warning and the GLFM suggestion. GFM would tell you to use GLFM instead, but after you make that change, Vale then says to use GitLab Flavored Markdown instead of GLFM. Now GFM tells you to use GitLab/GitHub Flavored Markdown. Had to add quotes to all the entries in that rule to make it render fine.

Issues that are easily resolved should be at the warning level, to emphasize that there is no reason to skip fixing it.

Substitution warnings:

• Before: 179 warnings in 1654 files.
• After: 841 warnings in 1654 files. (Not new warnings. Used to be at the suggestion level, now warning level)

Related issues

Related to #378718 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• 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.