Remove manually-entered milestone dates

What does this MR do and why?

This is 1 of 2 MRs that will close gitlab-docs#1337 (closed).

• Removes milestone date fields (announcement_date, removal_date, end_of_support_date) from the deprecations template (example.yml)
• Removes existing date field entries from deprecation yml files
• Replaces date fields on the template with an empty <span> tag. We'll populate dates from the gitlab-docs frontend (gitlab-docs!3415 (merged)) using a static JSON file.

This allows us to have a single source of truth* for milestone dates when shown on the Deprecations page. This clears up an issue we found with incorrect milestone dates. It also removes the need for deprecation authors to look up dates when adding a deprecation.

(* While the JSON file is an improvement over entering milestone dates on each individual deprecation, it was manually generated and could become incorrect if a future release happens on an unexpected date.)

Screenshots or screen recordings

This MR removes dates from the compiled page. The MR in gitlab-docs will re-add them (from a new source).

Before	After

How to set up and validate locally

1. Check out this branch
2. Test out adding a new deprecation, following the usual process, using the example.yml file
3. Verify you can compile deprecations: bin/rake gitlab:docs:compile_deprecations
4. Verify you can check deprecations: bin/rake gitlab:docs:check_deprecations
5. Visit the page in your local docs site. Deprecations should not include dates, but should be otherwise unchanged.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.