Add --silence-deprecation flag to mute mixed declaration Sass deprecation warnings

[pastelcyborg]

Done

• Added --silence-deprecation flag to all Sass build tasks to mute deprecation warnings

Fixes #5264
Fixes WD-13754

QA

• Checkout this branch
• Run any package.json build task involving Sass
• Witness that all deprecation warnings are now muted

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 release (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

Jenkins

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