0.31.0

Full diff at https://github.com/emilk/egui_plot/compare/0.30.0..HEAD

PRs

• Allow borrowing plot points via PlotPoints::Borrowed #64 by @mo8it and @bircni
• Add insertion_order and color_conflict_handling to Legend #65 by @Zoxc and @bircni
• Allow Plot::link_cursor to accept impl Into<Vec2b> #66 by @jetuk
• Axis: fix label thickness #68 by @jordens
• Update to egui 0.31.0 #72 by @Wumpf
• Update MSRV to Rust 1.81 #69 by @emilk