Update to no longer use ubuntu-20.04 runners

This preempts CI failures from the ubuntu-20.04 runners being deprecated
after 2025-04-01: actions/runner-images#11101.

.github/workflows/validate.yml

@@ -57,11 +57,7 @@ jobs:
           - python-version: "3.7"
             os: "macos-13"
           - python-version: "2.7"
-            os: "ubuntu-20.04"
-          - python-version: "3.5"
-            os: "ubuntu-20.04"
-          - python-version: "3.6"
-            os: "ubuntu-20.04"
+            os: "ubuntu-22.04"
           - python-version: "3.7"
             os: "ubuntu-22.04"
     runs-on: ${{ matrix.os }}
@@ -71,7 +67,10 @@ jobs:
       - uses: actions/checkout@v3
       - if: ${{ matrix.python-version == '2.7' }}
         run: |
-          sudo apt-get install python-is-python2
+          sudo add-apt-repository -y ppa:deadsnakes/ppa
+          sudo apt update
+          sudo apt install -y python2.7
+          sudo ln -s /usr/bin/python2.7 /usr/bin/python
           curl -sSL https://bootstrap.pypa.io/pip/2.7/get-pip.py -o get-pip.py
           python get-pip.py
         name: Set up Python ${{ matrix.python-version }} on ${{ matrix.os }}