Add MSRV change to upgrade guide

[findepi]

We sometimes add MSRV changes to upgrade guides. We should consistently either do it always, or not at all.
Let's add the recent change.

• follows Bump MSRV to 1.86.0 #17230

[adriangb]

Thank you!

[comphead]

Thanks @findepi the next good step would be update upgrading.md to reflect MSRV changes might be included

[adriangb]

Thanks @findepi the next good step would be update upgrading.md to reflect MSRV changes might be included

Do you mean like a meta section ## What you might find in upgrading.md?

[adriangb]

(I'm going to merge this and leave any general improvements other than upgrading.md itself for another PR)

[comphead]

Thanks @findepi the next good step would be update upgrading.md to reflect MSRV changes might be included

Do you mean like a meta section ## What you might find in upgrading.md?

Probably https://datafusion.apache.org/contributor-guide/api-health.html#upgrade-guides here would be the better place, saying not only code changes are included but also project level changes like MSRV