Use ubuntu-latest instead of ubuntu-16.04 (#497)

Author: taiki-e

.github/workflows/ci.yml

@@ -94,7 +94,7 @@ jobs:
           - powerpc-unknown-linux-gnu
           - powerpc64-unknown-linux-gnu
           - wasm32-unknown-unknown
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust