ci: use ubuntu-latest instead of ubuntu-20.04

Author: djc

.github/workflows/build.yml

@@ -22,7 +22,7 @@ jobs:
           - stable
           - beta
           - nightly
-        os: [ubuntu-20.04]
+        os: [ubuntu-latest]
         # but only stable on macos/windows (slower platforms)
         include:
           - os: macos-latest
@@ -93,7 +93,7 @@ jobs:
         run: cargo test --release --no-run
 
   msrv:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
@@ -122,7 +122,7 @@ jobs:
 
   docs:
     name: Check for documentation errors
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4