Figma: deprecate theme libraries

[laurenmrice]

This is the umbrella issue for deprecating the Carbon theme libraries in Figma, in favor of keeping the White theme library (Renaming to "All themes") and implementing the theming variable feature that was released during the Config conference.

Tasks

Beta Give feedback

1. Figma themes deprecation: Update Figma v11 libraries #704
   kit: figma role: design ✏️ type: enhancement 💡 +3
   
2. Figma themes deprecation: Update Figma v10 libraries #710
   kit: figma role: design ✏️ type: enhancement 💡 +3
   
3. Figma themes deprecation: Update Website docs carbon-website#3644
   role: design ✏️ type: docs 📖 type: enhancement 💡 +3
   
4. Figma themes deprecation: Slack announcement #706
   role: design ✏️ type: docs 📖 +2
   
5. Publish Figma library and Website doc updates
6. Figma themes deprecation: Leftover kit work #712
   kit: figma role: design ✏️ type: bug 🐛 +3
   
7. Figma theme variables: Medium article carbon-website#3653
   content: copy ✍️ role: design ✏️ +2