Raise minimum tested toolchain to rust 1.61

Required by the transitive dependency on hashbrown.

    error: package `hashbrown v0.13.1` cannot be built because it
    requires rustc 1.61.0 or newer, while the currently active rustc
    version is 1.60.0

Author: dtolnay

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.59.0]
+        rust: [nightly, beta, stable, 1.61.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3