MongoDB: Fix CI

- Downgrade to Python 3.11
- Downgrade to pymongo<4 on MongoDB 2
- Skip testing against MongoDB 2, because `auth failed`

.github/workflows/mongodb.yml

@@ -36,8 +36,8 @@ jobs:
       fail-fast: false
       matrix:
         os: ["ubuntu-latest"]
-        python-version: ["3.8", "3.12"]
-        mongodb-version: ["2", "3", "4", "5", "6", "7"]
+        python-version: ["3.8", "3.11"]
+        mongodb-version: ["3", "4", "5", "6", "7"]
 
     env:
       OS: ${{ matrix.os }}
@@ -70,6 +70,11 @@ jobs:
         # Install package in editable mode.
         pip install --use-pep517 --prefer-binary --editable=.[mongodb,test,develop]
 
+    - name: Downgrade pymongo on MongoDB 2
+      if: matrix.mongodb-version == '2'
+      run: |
+        pip install 'pymongo<4'
+
     - name: Run linter and software tests
       run: |
         pytest -m mongodb