chore: changelog for 0.28.0

[SteveLauC]

What does this PR do

Prepare for a new release 0.28.0, and edit the changelog for it.

If a new release is allowed, then I will:

1. Update the version number in Cargo.toml
2. Run cargo release
3. Push the above changes directly to the master branch

Since this is a one-way trip, hi @asomers, is there anything you want to do before 0.28.0?

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

Is there anything left that you'd like me to do on #2269 first?

[SteveLauC]

Is there anything left that you'd like me to do on #2269 first?

There are 2 requested changes in #2269, would you like to resolve them?

[Jan561]

Please remember to remove this change before releasing a new version, since this would be a regression of safety (#2120 fixed a safety bug but embarrassingly also introduced a new one, as explained in #2162, revert in #2227).

I'll revisit #2227 later this day and probably make some slight changes to better match the previous version regarding some elided lifetimes (my version should be equivalent, but I should have opened a separate PR to remove the additional lifetimes that became redundant in recent compiler versions instead of doing both things in one PR).

[SteveLauC]

Please remember to remove this change before releasing a new version

Will do it when updating this PR:)

[Jan561]

Done