DOC: Drop all mentions of CUDA 11

[leofang]

It seems we have a few places that still teach users about CUDA 11. We already drop CUDA 11 support, and only cover CUDA 12/13. Need to update the docs throughout the codebase.

[leofang]

@copilot please update the installation guides and support policies to make sure we cover CUDA 12 and 13 instead of 11 and 12. Leave the historical documents (ex: release notes) intact.