Remove outdated Suru strips from the website

[ClementChaumel]

Done

Removed the suru strips from the index page and other templates

I kept the current accent colour.

QA

• Open demo
• check https://vanilla-framework-4752.demos.haus/accessibility
• See that the suru strips are gone and that the alternative looks good

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.

Screenshots

[webteam-app]

Demo starting at https://vanilla-framework-4752.demos.haus

[bartaz]

I think we can drop the custom colours (including orange), as it was only needed for suru.

https://github.com/canonical/vanilla-framework/blob/9a4fb40c64aa58b6adc19b296971815bd95781c9/scss/docs/site.scss#LL5C1-L14C57

Let's use Vanilla grey defaults instead.