Update the msrv to 1.48.

Author: sunfishcode

.clippy.toml

@@ -1 +1 @@
-msrv = "1.46"
+msrv = "1.48"

.github/workflows/ci.yml

@@ -89,7 +89,7 @@ jobs:
       matrix:
         # When updating this, the reminder to update the minimum supported
         # Rust version in Cargo.toml and .clippy.toml.
-        rust: ['1.46']
+        rust: ['1.48']
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust