3.0 Migration guide should mention removed sf::Keyboard::Key aliases

[redinator2000]

sf::Keyboard::Key enumeration removed some alias values, that according to a comment, were depreciated in 2.6.x https://github.com/SFML/SFML/commit/756a399909a913364de00742afce6b91441646b9

This is a breaking change, and should be mentioned in the migration guide https://www.sfml-dev.org/tutorials/3.0/getting-started/migrate/

I suggest a list of all the depreciated values somewhere in the document, so it is easy to ctrl+f "Dash", find what it was aliasing, and easily fix the compiler error