releases: increase MSRV to 1.74.0

We cannot build `releases` with the existing lockfile because its MSRV
is too high. If we intend the MSRV to be lower then we need to fix the
lockfile and reduce the MSRV in `cargo-rbmt`.

Author: apoelstra

Cargo.lock

@@ -9,7 +9,7 @@ categories = []
 keywords = []
 readme = "README.md"
 edition = "2021"
-rust-version = "1.56.1"
+rust-version = "1.74.0"
 
 [dependencies]
 anyhow = { version = "1.0.82", default-features = false, features = ["std"] }