rust-1.90: Update MSRV to 1.84.0

We already required at least 1.83.0 since we're using `Option::unwrap`
in const context in a few places, but rust-1.90 now correctly points
this out and emits an error.

RHEL 9.6 is on 1.84.0 so bump to match that.

Signed-off-by: John Eckersberg <[email protected]>

Author: jeckersb

crates/lib/Cargo.toml

@@ -8,8 +8,7 @@ repository = "https://github.com/bootc-dev/bootc"
 # project isn't actually published as a crate.
 version = "1.8.0"
 # In general we try to keep this pinned to what's in the latest RHEL9.
-# However right now, we bumped to 1.82 as that's what composefs-rs uses.
-rust-version = "1.82.0"
+rust-version = "1.84.0"
 
 include = ["/src", "LICENSE-APACHE", "LICENSE-MIT"]