ci: use latest ubuntu

Author: MatthewPattell

.github/workflows/pr-check.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   check:
     name: Check PR
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v2

.github/workflows/release.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   release:
     name: Release
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
       - name: Checkout