Add advisory indicating mbox is unmaintained

[kcking]

No description provided.

[amousset]

Thanks for reporting! As per our unmaintained advisory policy, could you create an issue on the mbox repo asking if the author agrees with opening an unmaintained advisory?

[CommanderStorm]

@kcking
The issue you are using as a reference have been resolved

• malloc() on Windows cannot be sound for aligned types kennytm/mbox#25
• reimplement the gen_{malloc,free,realloc} functions to remove dangling() kennytm/mbox#26

=> calssiying the crate as unmaintained does not reflect the current state
=> This PR can be closed or re-framed as unsoundness issues (one per issue is likely better) in a past versions of the crate

[amousset]

@CommanderStorm thanks for the update, closing this PR.

Feel free to open a new one for the unsoundness issues.