ci: switch linux hosts to ubuntu-latest

This is what we prefer in the main rustls repo.

.github/workflows/build.yml

@@ -3,23 +3,23 @@ name: rustls-pemfile
 
 jobs:
   rustfmt:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt
       - uses: actions/checkout@v4
       - run: cargo fmt --all -- --check
   clippy:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
       - uses: actions/checkout@v4
       - run: cargo clippy --locked --all-features --all-targets
   rustdoc:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: dtolnay/rust-toolchain@stable
       - uses: actions/checkout@v4
@@ -34,7 +34,7 @@ jobs:
           - stable
           - beta
           - nightly
-        os: [ubuntu-20.04]
+        os: [ubuntu-latest]
         # but only stable on macos/windows (slower platforms)
         include:
           - os: macos-latest