Upgrade MSRV to 1.85

[notmandatory]

We should update the bdk_wallet 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 will resolve #101.

To support enterprise users I think we should continue to support as an MSRV what ever is released with Debian stable (see #203).

Additional context

Changing our MSRV is also a good time to implement the suggestion in #79.

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

[tnull]

Might make sense to wait a few weeks/months after the Trixie release, but otherwise concept ACK!

[notmandatory]

The BDK 3.0 milestone isn't scheduled until October so we can wait a couple months to do this.