Upgrade to Rust 1.85+ when able

[JoeCap08055]

Feature Description

Details

Recent upstream crates are causing the following error in CI:

error: failed to parse manifest at `/Users/joecaputo/.cargo/registry/src/index.crates.io-6f17d22bba15001f/base64ct-1.8.0/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 (1.84.1 (66221abde 2024-11-19)).
  Consider trying a newer version of Cargo (this may require the nightly release).
  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2024 for more information about the status of this feature.

Root cause: In our CI pipeline, we run cargo generate-lockfile before running cargo deny. This generates the following output:

    Updating git repository `https://github.com/paritytech/polkadot-sdk`
    Updating crates.io index
     Locking 1292 packages to latest compatible versions
      Adding apache-avro v0.17.0 (available: v0.18.0)
      Adding base64ct v1.8.0 (requires Rust 1.85)
      Adding derive_more v0.99.20 (available: v2.0.1)
      Adding docify v0.2.9 (available: v0.4.1)
      Adding hex-literal v0.4.1 (available: v1.0.0, requires Rust 1.85)
      Adding impl-serde v0.4.0 (available: v0.5.0)
      Adding jsonrpsee v0.24.9 (available: v0.25.1, requires Rust 1.85.0)
      Adding numtoa v0.2.4 (available: v0.3.0)
      Adding proc-macro-crate v1.1.3 (available: v1.3.1)
      Adding serial_test v2.0.0 (available: v3.2.0)
      Adding twox-hash v1.6.3 (available: v2.1.0)
      Adding unicode-normalization v0.1.22 (available: v0.1.24)

Of particular note, the offending package, base64ct has only one version available, which requires Rust 1.85.

Temporary solution in place: use +nightly-2025-04-03 when running cargo deny in CI; remove once we've upgraded to Rust 1.85+

Searched for Related Issues

• [x] I have done a search for related issues and either found none, or noted them

[JoeCap08055]

Note, as of this time, upstream polkadot-sdk@stable-2503 crates are only validated against Rust 1.84.1