Decide Official MSRV Policy for WhoAmI 2.x

[AldaronLau]

Potential options:

• Crash on illumos due to different passwd struct definition #91 (comment)
• https://github.com/ardaku/whoami/tree/d6ee13ed9e818aa51b8d86d95e8009a376289a40?tab=readme-ov-file#msrv

[AldaronLau]

I wrote up draft MSRV guidelines to use across all crates I maintain (probably needs some editing):

https://github.com/ardaku/.github/blob/05a83745d9366a78da6c1680256c3fc76a9f50d3/profile/MSRV.md

It's somewhere between the two above, with more details. I think my original goal of 3 years support is unmaintainable in the long run, which is why I lowered it to 18 months, which I think is a reasonable enough compromise.