1.28.0 release planning

[djc]

As discussed in #3985 (comment), starting an issue for this because it makes it easier to have a discussion in one place on what items should be in the milestone(s).

I would propose that we remove these from the 1.28.0 milestone:

• rustup-init leaves user with colored terminal #2590
• rustup self uninstall fails on Windows if a custom toolchain is installed #2789
• rustup set default-host does not change the default toolchain #3651