Update MSRV to 1.85

[notmandatory]

We should update the bdk crates MSRV to 1.85 with rust edition 2024. The rust 1.85 version is included in the next Debian stable release 13 (trixie) which should be released this week.

Use case

Upgrading the MSRV should resolve #1949 and #1814.

Additional context

This should be targeted to the bdk_wallet 3.0 milestone, see bitcoindevkit/bdk_wallet#289.

https://lists.debian.org/debian-devel-announce/2025/07/msg00003.html
https://packages.debian.org/search?keywords=rust