docs: Specify minimum supported rust version (MSRV) to 1.59

[tomocrafter]

Description

starship now uses std::thread::available_parallelism which stablized at Rust version 1.59.
https://github.com/starship/starship/blob/master/src/main.rs#L235
therefore, This PR sets MSRV to 1.59 to make sure everyone can build.

Motivation and Context

Closes #3716
Closed #3719

Screenshots (if appropriate):

How Has This Been Tested?

• I have tested using MacOS
• I have tested using Linux
• I have tested using Windows

Checklist:

• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[tomocrafter]

I apologize that I haven't seen this PR #3719. closing this PR.

[andytom]

It looks like #3719 has stalled a bit so feel free to reopen this PR. I think all you need to do is run dprint fmt to fix the formatting issues.

[davidkna]

Please also add a comment that the MSRV is only intended as a hint and that only the latest version is officially supported.
CI testing is currently not looking at older rust versions at all, so this declaration is likely to break in the future without further changes.