Update text colour to 000

[bartaz]

Done

Updates default text colour to be black #000 instead of dark grey #111

Fixes WD-4561
Fixes #4784

QA

• Open demo
• Make sure all default text is black.
• Review updated documentation:
  • Check that migration guide mentions the colour change: https://vanilla-framework-4786.demos.haus/docs/migration-guide-to-v4#updated-text-colour

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix relesase (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

[webteam-app]

Demo starting at https://vanilla-framework-4786.demos.haus

[ClementChaumel]

Good work 👌️