Use any Python >= 2.7.16 for CI

Azure agents have been upgraded to 2.7.17, breaking C++ CI. To
future-proof CI, change the version spec to accept any Python 2.7
version beyond 2.7.16, the originally validated one.

(cherry picked from commit 92bb63f)

tools/ci/templates/jobs-cpp.yaml

@@ -44,11 +44,11 @@ jobs:
       nugetConfigPath: '$(NuGetConfigPath)'
     timeoutInMinutes: 30
 
-  # Ensure that Python 2.7.16 is the default; the Google scripts don't work with Python 3.x
+  # Ensure that Python 2.7.16+ is the default; the Google scripts don't work with Python 3.x
   - task: UsePythonVersion@0
-    displayName: 'Use Python 2.7.16 x64 for Google GN'
+    displayName: 'Use Python 2.7.16+ x64 for Google GN'
     inputs:
-      versionSpec: 2.7.16
+      versionSpec: '>=2.7.16 <2.8.0'
     timeoutInMinutes: 5
 
   # Map build* variables to script* ones