Remove support for ubuntu-20.04

* It's gone: actions/runner-images#11101

Author: eregon

README.md

@@ -49,7 +49,7 @@ The action works on these [GitHub-hosted runners](https://docs.github.com/en/act
 
 | Operating System | Supported |
 | ---------------- | --------- |
-| Ubuntu  | `ubuntu-20.04`, `ubuntu-22.04`, `ubuntu-24.04`, `ubuntu-22.04-arm`, `ubuntu-24.04-arm` |
+| Ubuntu  | `ubuntu-22.04`, `ubuntu-24.04`, `ubuntu-22.04-arm`, `ubuntu-24.04-arm` |
 | macOS   | `macos-13` and newer versions |
 | Windows | `windows-2019`, `windows-2022` |
 

common.js

@@ -171,7 +171,6 @@ export async function hashFile(file) {
 
 // macos is not listed explicitly, see below
 const GitHubHostedPlatforms = [
-  'ubuntu-20.04-x64',
   'ubuntu-22.04-x64',
   'ubuntu-22.04-arm64',
   'ubuntu-24.04-x64',