Raise minimum rustc version in CI to 1.52

Required by the time crate:

    error[E0658]: the `unsafe_op_in_unsafe_fn` lint is unstable
       --> github.com-1ecc6299db9ec823/time-0.3.6/src/lib.rs:88:1
        |
    88  | / #![deny(
    89  | |     anonymous_parameters,
    90  | |     clippy::all,
    91  | |     const_err,
    ...   |
    102 | |     unused_extern_crates
    103 | | )]
        | |__^

Author: dtolnay

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.51.0]
+        rust: [nightly, beta, stable, 1.52.0]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master