Raise required compiler to rust 1.60

On older ones, cargo fails like this:

    error: failed to select a version for the requirement `syn = "^2.0.59"`
    candidate versions found which didn't match: 2.0.56, 2.0.55, 2.0.54, ...
    location searched: crates.io index
    required by package `monostate-impl v0.1.12`

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
  strategy:
  fail-fast: false
  matrix:
- rust: [nightly, beta, stable, 1.56.0]
+ rust: [nightly, beta, stable, 1.60.0]
  timeout-minutes: 45
  steps:
  - uses: actions/checkout@v4

Cargo.toml

@@ -9,7 +9,7 @@ edition = "2021"
 keywords = ["serde", "serialization"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/monostate"
-rust-version = "1.56"
+rust-version = "1.60"
 
 [dependencies]
 monostate-impl = { version = "=0.1.11", path = "impl" }

impl/Cargo.toml

@@ -6,7 +6,7 @@ description = "Implementation detail of the monostate crate"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/monostate"
-rust-version = "1.56"
+rust-version = "1.60"
 
 [lib]
 proc-macro = true