Update azure pipelines to gcc-12/g++-12

The new `ubuntu-latest` image based on Ubuntu 24.04 has gcc-12 and does
not have gcc-9. This updates to a more recent gcc.

Note: while the Azure images have gcc-14 as well, I've chosen gcc-12
because it is the latest version of gcc available in 24.04's apt
repositories. To get newer versions you need to poke at external apt
repositories.

Author: llvm-beanz

azure-pipelines.yml

@@ -73,13 +73,13 @@ stages:
         Linux_Gcc_Release:
           image: ${{ variables.linux }}
           configuration: Release
-          CC: gcc-9
-          CXX: g++-9
+          CC: gcc-12
+          CXX: g++-12
         Linux_Gcc_Debug:
           image: ${{ variables.linux }}
           configuration: Debug
-          CC: gcc-9
-          CXX: g++-9
+          CC: gcc-12
+          CXX: g++-12
         MacOS_Clang_Release:
           image: ${{ variables.macOS }}
           configuration: Release