Stop testing compilers older than 1.68

Cargo 1.68 has the sparse registry protocol which greatly speeds up CI.
Testing compilers older than that is too slow for no benefit.

Author: dtolnay

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.56.0]
+        rust: [nightly, beta, stable, 1.68.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4

Cargo.toml

@@ -9,7 +9,7 @@ edition = "2018"
 keywords = ["serde", "serialization", "no_std", "bytes"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/serde-rs/bytes"
-rust-version = "1.53"
+rust-version = "1.68"
 
 [features]
 default = ["std"]