CI: remove ubuntu-20.04; move tar_gz to -latest

ubuntu-20.04 is being retired by Github
tar_gz is mainly to verify URLs, OS version doesn't matter

.github/workflows/modified_scripts_build.yml

@@ -157,7 +157,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ${{fromJson(needs.discover_modified_scripts.outputs.versions)}}
-        os: ["ubuntu-20.04", "ubuntu-22.04", "ubuntu-24.04"]
+        os: ["ubuntu-22.04", "ubuntu-24.04"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
@@ -215,7 +215,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ${{fromJson(needs.discover_modified_scripts.outputs.versions_cpython_only)}}
-        os: ["ubuntu-20.04", "ubuntu-22.04", "ubuntu-24.04"]
+        os: ["ubuntu-latest"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4

.github/workflows/pyenv_tests.yml

@@ -12,7 +12,6 @@ jobs:
         os:
           - ubuntu-24.04
           - ubuntu-22.04
-          - ubuntu-20.04
           - macos-14
           - macos-13
     runs-on: ${{ matrix.os }}