Retire Ubunu Impish, because issues with C++20 support

See https://bugs.launchpad.net/mixxx/+bug/1980639

CMakeLists.txt

@@ -153,7 +153,7 @@ endif()
 
 #######################################################################
 
-set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD 17)
 if(MSVC)
   # Ensure MSVC populates __cplusplus correctly.
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Zc:__cplusplus")
@@ -2985,7 +2985,7 @@ if (NOT CPACK_DEBIAN_PACKAGE_RELEASE)
    set(CPACK_DEBIAN_PACKAGE_RELEASE 1)
 endif()
 
-set(CPACK_DEBIAN_DISTRIBUTION_RELEASES focal impish jammy kinetic)
+set(CPACK_DEBIAN_DISTRIBUTION_RELEASES focal jammy kinetic)
 set(CPACK_DEBIAN_SOURCE_DIR ${CMAKE_SOURCE_DIR})
 set(CPACK_DEBIAN_UPLOAD_PPA_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/packaging/CPackDebUploadPPA.cmake")
 set(CPACK_DEBIAN_INSTALL_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/packaging/CPackDebInstall.cmake")