Cargo.toml: MSRV 1.85 → 1.90 (matches Dockerfile floor)

CI's MSRV job caught another transitive dep floor:

    error: rustc 1.85.1 is not supported by the following packages:
      darling@0.24.1 requires rustc 1.88.0
      icu_collections@2.3.0 requires rustc 1.88
      ...

Dockerfile already pins `rust:1.90-bookworm` for exactly this
reason (transitive deps via ratatui + darling required 1.88+).
Align the declared workspace MSRV with the same floor so the
declaration matches reality.

Previously bumped 1.80 → 1.82 (own-code floor) → 1.85 (zeroize
edition2024) → now 1.90 (darling + icu). CI's msrv job is
literally its purpose: keeping the declaration honest.

Users with any Rust older than 1.90 get a clean "rustc X is not
supported" from cargo instead of a mysterious dep-parse error.

Author: BootIntel Agent

Cargo.toml

@@ -21,7 +21,7 @@ resolver = "2"
 [workspace.package]
 version = "0.2.0"
 edition = "2021"
-rust-version = "1.85"
+rust-version = "1.90"
 license = "Apache-2.0"
 authors = ["Zenofex LLC <hello@bootintel.com>"]
 repository = "https://github.com/bootintel/cli"