Remove CUDA 11 from code, docs, etc.

[jakirkham]

Description

CUDA 11 is being removed in RAPIDS 25.08 as noted in the RSN & issue below.

CI and package builds for CUDA 11 have already been dropped.

However references in the code, docs, etc. are still present.

This issue tracks removing all remaining CUDA 11 references.

References:

• RSN 48
• https://github.com/rapidsai/build-planning/issues/184