Updated roadmap in about.md

[edhamiltonHO]

This will close #302

Content change

• [x] Please review the accessibility checks for content changes.

I can confirm:

• [x] Content does not include any code or configuration changes (excluding frontmatter information)
• [x] Content meets the content standards e.g. Writing a principle and Writing a standard
• [x] Content is suitable to open source, i.e.:
  • Content does not relate to unreleased gov policy
  • Content does not refer to anti-fraud mechanisms
  • Content does not include sensitive business logic
• [x] Last updated date for content is correct