MAINT, CI: Update Ubuntu 18.04 to Ubuntu 20.04

.github/workflows/build_test.yml

@@ -80,8 +80,8 @@ jobs:
 
   old_gcc:
     needs: [smoke_test]
-    # provides GCC 6, 7, 8
-    runs-on: ubuntu-18.04
+    # provides GCC  7, 8
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -99,14 +99,7 @@ jobs:
         pythonx -m pip install --upgrade pip setuptools wheel
         pythonx -m pip install -r test_requirements.txt
     - name: Install Compilers
-      run: sudo apt install g++-6 g++-7 g++-8 -y
-    - name: Build gcc-6
-      run: |
-        export CC=/usr/bin/gcc-6
-        export CXX=/usr/bin/g++-6
-        pythonx setup.py install --user
-    - name: Runtests gcc-6
-      run: pythonx runtests.py -n
+      run: sudo apt install g++-7 g++-8 -y
     - name: Build gcc-7
       run: |
         export CC=/usr/bin/gcc-7
@@ -199,11 +192,14 @@ jobs:
 
   full:
     needs: [smoke_test]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       USE_WHEEL: 1
       RUN_FULL_TESTS: 1
-      RUN_COVERAGE: 1
+      # The coverage option fails with internal compiler error
+      # in coverage.c with Ubunto 20.04. That problem is fixed
+      # in 22.04, but there are other, new errors.
+      #RUN_COVERAGE: 1
       INSTALL_PICKLE5: 1
     steps:
     - uses: actions/checkout@v3

azure-pipelines.yml

@@ -74,7 +74,7 @@ stages:
   - job: Lint
     condition: and(succeeded(), eq(variables['Build.Reason'], 'PullRequest'))
     pool:
-      vmImage: 'ubuntu-18.04'
+      vmImage: 'ubuntu-20.04'
     steps:
     - task: UsePythonVersion@0
       inputs:

doc/release/upcoming_changes/22598.compatibility.rst

@@ -0,0 +1,5 @@
+NumPy builds are no longer tested on GCC-6
+------------------------------------------
+Ubuntu 18.04 is deprecated for GitHub actions and GCC-6 is not available on
+Ubuntu 20.04, so builds using that compiler are no longer tested. We still test
+builds using GCC-7 and GCC-8.