Rename all background{n} tokens as bg{n}

[aweell]

Description

Rename all tokens using background as bg to reduce the name length.

• Update all values for all brands
• Update all documentation references

Tasks

• [ ] Update specs
• [ ] Design documentation
• [ ] Add to mobile library
• [ ] Add to desktop library
• [ ] Add to UI Kit
• [ ] Android ticket
• [ ] iOS ticket
• [ ] Web ticket

[yceballost]

apply to backgroundAlternative for example? bgAlternative? what about background = bg?