JuMP v1.21.1

Diff since v1.21.0

Merged pull requests:

• Improve docstrings of backend and optimizer_index (#3719) (@odow)
• [docs] add bounds to big-M examples (#3720) (@odow)
• [docs] fix bounds in big-M examples of tips_and_tricks.jl (#3721) (@odow)
• [docs] update to vale.sh v3.0.5 (#3722) (@odow)
• Fix adjoint of GenericNonlinearExpr (#3724) (@odow)
• Prep for v1.21.1 (#3726) (@odow)

Closed issues:

• Get attributes set by optimizer without calling optimize (#3684)
• Y' when Y is of type ::Matrix{NonlinearExpr} (#3723)