Update manifest-v3.md to remove "Chromium timeline is Jan 2024"

[mikehoffms]

Rendered article section for review:

• Overview and timelines for migrating to Manifest V3
  • https://review.learn.microsoft.com/microsoft-edge/extensions-chromium/developer-guide/manifest-v3?branch=pr-en-us-3076
  • Public rendered article in PR
  • Before/Live - 3rd paragraph from end

This PR makes the following changes:

• Removed outdated Chromium date (instead of trying to update it like PR 3041 attempted to do).
  • Consistently used TBD.
• Writer/Editor pass on article:
  • Structural edit; eg added headings.
  • Copyedit: eg changed future tense to past/present tense, fixed grammar.
• Updated external links to not redirect.
  • Updated link text to match destination titles or headings.

This PR replaces PR #3041.

AB#49108231

[learn-build-service-prod]

Learn Build status updates of commit c726a70:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/extensions-chromium/developer-guide/manifest-v3.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel