build(msrv): ⬆️ bump MSRV since Rust 1.80 is released

Features from Rust 1.78 can be used now.

Notable features from [Rust 1.78](https://releases.rs/docs/1.78.0/):

- Concrete signature for AFIT (rust-lang/rust#120103)
- `impl Read for &Stdin`

.vscode/settings.json

@@ -4,6 +4,7 @@
     "config",
     "deps",
     "devcontainer",
+    "msrv",
     "packaging",
     "rsjudge-amqp",
     "rsjudge-grpc",

Cargo.toml

@@ -20,7 +20,7 @@ license = "Apache-2.0"
 repository = "https://github.com/NJUPT-SAST/rsjudge"
 
 # MSRV is set to N - 2, where N is the current stable version.
-rust-version = "1.77"
+rust-version = "1.78"
 
 [workspace.dependencies]
 log = "0.4.22"