Switch to ubuntu-20.04 runners

Author: lnicola

.github/workflows/release.yaml

@@ -31,13 +31,13 @@ jobs:
           - os: windows-latest
             target: aarch64-pc-windows-msvc
             code-target: win32-arm64
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             target: x86_64-unknown-linux-gnu
             code-target: linux-x64
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             target: aarch64-unknown-linux-gnu
             code-target: linux-arm64
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             target: arm-unknown-linux-gnueabihf
             code-target: linux-armhf
           - os: macos-11