build: Bump MSRV to 1.89.0

This is required to support exclusive locking on files which is needed
for safe test ID generation when using nextest (since it runs each test
as a separate process.)

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

Cargo.toml

@@ -15,7 +15,7 @@ version = "49.0.0"
 # a.) A dependency requires it,
 # b.) If we want to use a new feature and that MSRV is at least 6 months old,
 # c.) There is a security issue that is addressed by the toolchain update.
-rust-version = "1.88.0"
+rust-version = "1.89.0"
 
 [profile.release]
 codegen-units = 1