point to v1.89.0 of the rust toolchain to resolve edition2024 build failure in CI

[ben-p-commits]

Fix: Update Rust toolchain to 1.85.0

A CI build associated with another PR was failing the clippy job due to a transitive dependency (crypto-common v0.2.0-rc.5) requiring Cargo's edition2024 feature, which was not stabilized until Rust 1.85.0.

error: failed to parse manifest at `crypto-common-0.2.0-rc.5/Cargo.toml`
Caused by:
  feature `edition2024` is required
  The package requires the Cargo feature called `edition2024`, but that feature
  is not stabilized in this version of Cargo (1.81.0)

Per the Rust docs, edition2024 requires Rust 1.85.0+.

Furthermore, we actually need to bump to 1.89.0 due to an additional error (when attempting to run with 1.85.0):

error: rustc 1.85.0 is not supported by the following packages:
  automerge@0.7.2 requires rustc 1.89.0
  smol_str@0.3.4 requires rustc 1.89
Either upgrade rustc or select compatible dependency versions with
`cargo update <name>@<current-ver> --precise <compatible-ver>`
where `<compatible-ver>` is the latest version supporting rustc 1.85.0

Changes

Updated the pinned Rust toolchain from 1.81.0 to 1.89.0 in all three CI jobs (automerge-dependencies, rustfmt, clippy).

[ben-p-commits]

@alexjg , @heckj - not sure who has the ability to merge PRs. assuming it's one of you guys. Would you have the bandwidth to do so to help unblock my follow-up PR: #224 ?