Pin rustc and toolchain to 1.86

[thomasp85]

Fix #469

[teunbrand]

Mostly LGTM but I'm a bit bothered by having to mix different versions in the same build/publish workflows. Is there a reason we must keep adbc_datafusion >= 53.1.0 and cannot downgrade this version to be compatible with the rest?

[thomasp85]

We can't avoid te split for WASM, and forcing the rust version there is also non-sensical since wasm doesn't matter for cran. As for the adbc it is sadly a game of whack-a-mole since adbc depend on crates that have silent 1.88 requirements... we would have to pin all of these as well to get us there and then carry that burden onwards

[eitsupi]

adbc depend on crates that have silent 1.88 requirements

I think this may be a misunderstanding.
The MSRV of adbc_core 0.23.0 is 1.81.0.
https://github.com/apache/arrow-adbc/blob/efdc10ba93f7ae31a7beeb801c13fdcd15f01c6e/rust/Cargo.toml#L26

This has been rigorously tested on CI and is fully guaranteed.

If you're struggling with MSRV for transitional dependencies, I think it's worth considering using Cargo resolver 3.
Resolver 3 handles dependency resolution with MSRV in mind.
https://doc.rust-lang.org/edition-guide/rust-2024/cargo-resolver.html

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Also, please note that adbc_datafusion may be removed from crates.io.
If the DataFusion build is causing constraints, I recommend rewriting the tests.
apache/arrow-adbc#4377 (comment)