Update macos/linux builds to *-latest (#4815)

This updates the ADO yaml to select the latest version of the OS. In the
case of MacOS, this will still be 11 for now. For Ubuntu, it will
silence the warnings that we are using a deprecated version and save us
the headache of having to do this again.

The risk is that a new version of either OS will cause the build to
break without our intervention, but the real motivation for explicit
versions was an earlier impulse to support the minimal build system that
is no longer a priority. As for the risk, I feel it is minimal and we
can respond in a timely fashion if it should occur.

azure-pipelines.yml

@@ -43,8 +43,8 @@ stages:
     timeoutInMinutes: 90
 
     variables:
-      macOS: macOS-11
-      linux: Ubuntu-18.04
+      macOS: macOS-latest
+      linux: Ubuntu-latest
 
     strategy:
       matrix:
@@ -64,14 +64,14 @@ stages:
         Linux_Gcc_Release:
           image: ${{ variables.linux }}
           configuration: Release
-          CC: gcc-7
-          CXX: g++-7
+          CC: gcc-9
+          CXX: g++-9
           CXX_FLAGS:
         Linux_Gcc_Debug:
           image: ${{ variables.linux }}
           configuration: Debug
-          CC: gcc-7
-          CXX: g++-7
+          CC: gcc-9
+          CXX: g++-9
           CXX_FLAGS:
         MacOS_Clang_Release:
           image: ${{ variables.macOS }}