Populate deprecation dates from JSON file

What does this MR do and why?

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

• Adds a JSON file containing milestones with their release dates
• Uses this data to populate milestone date information on the Deprecations page

This replaces dates that were previously added to each individual deprecation (and occasionally wrong).

Screenshots, screen recordings, or links to review app

No visual differences from the current page:

Before	After

But dates are now pulled from release_dates.json.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Requires gitlab!108362 (merged) to test. Until that one is merged, we'll continue to just see dates from the individual deprecation entries, and the code introduced here won't do anything. So this is not testable on the Review App.

To test locally:

1. Set up a local docs environment
2. If gitlab!108362 (merged) is not merged yet: In your gitlab repo, check out the sarahgerman/deprecation-dates branch
3. Compile and run the site: make view
4. Verify that milestone dates appear on the Deprecations page

Merge request 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 merge request.