Failing CI: Prost requires Rust 1.71.1

[gstvg]

Describe the bug

Prost bumped it's MSRV from 1.70 to 1.71.1 on it's v0.13.3 release
This clashes with arrow-flight MSRV 1.70, causing CI errors

Additional info

The 1.70 MSRV was set on #4490, on Jul 8, 2023, more than one year ago
Rust 1.70 was released on 1 June, 2023, while 1.71.1 on 3 August, 2023: ~2 months of difference

I think we can safely bump arrow-flight MSRV from 1.70 to 1.71.1, otherwise pin prost to v0.13.2.

[alamb]

label_issue.py automatically added labels {'arrow'} from #6437

[alamb]

label_issue.py automatically added labels {'arrow-flight'} from #6437

[alamb]

label_issue.py automatically added labels {'documentation'} from #6434