chore(build): tune release profile and QR dependency (#6660)

Switch IAM QR rendering from qrcode to qrcode-rs 2.0.0 while keeping only the std and svg feature path enabled.

Set the release profile to a single codegen unit and disable release debuginfo as requested.

Verification:

- cargo info qrcode-rs --registry crates-io

- cargo tree -p rustfs-iam -e features

- CARGO_TARGET_DIR=/private/tmp/rustfs-target-qrcode-rs-profile-tuning cargo test -p rustfs-iam --locked

- cargo fmt --all --check

- git diff --check

Co-authored-by: heihutu <heihutu@gmail.com>

crates/iam/src/mfa/qr.rs

@@ -20,7 +20,7 @@
 //! Unicode block art (for a terminal), and the console needs no new npm
 //! dependency to show a code.
 
-use qrcode::{EcLevel, QrCode, render::svg, render::unicode};
+use qrcode_rs::{EcLevel, QrCode, render::svg, render::unicode};
 
 #[derive(Debug, thiserror::Error, PartialEq, Eq)]
 pub enum QrError {