Update Linux CI to Ubuntu 20.04 (#7015)

- Switch to Ubuntu 20.04 Docker image ghcr.io/lmms/linux.gcc:20.04
- Linux packages have migrated from Docker Hub to https://github.com/orgs/lmms/packages
- Built using the Dockerfiles from Update Linux images lmms-ci-docker#15
- Updated the veal submodule to the latest commit on the default ladspa branch
- Fixed an error when catching a polymorphic type with GCC 9. See: LMMS/veal@0ae9287
- Added GCC flag -Wno-format-truncation for ZynAddSubFx build.
- Adds GCC flag -Wno-format-overflow for calf/veal build.

Closes #6993

plugins/LadspaEffect/calf/CMakeLists.txt

@@ -35,10 +35,12 @@ SET_TARGET_PROPERTIES(veal PROPERTIES PREFIX "")
 TARGET_COMPILE_DEFINITIONS(veal PRIVATE DISABLE_OSC=1)
 
 SET(INLINE_FLAGS "")
+SET(OTHER_FLAGS "")
 IF("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
 	SET(INLINE_FLAGS -finline-functions-called-once -finline-limit=80)
+	SET(OTHER_FLAGS -Wno-format-overflow)
 ENDIF()
-target_compile_options(veal PRIVATE -fexceptions -O2 -finline-functions ${INLINE_FLAGS})
+target_compile_options(veal PRIVATE -fexceptions -O2 -finline-functions ${INLINE_FLAGS} ${OTHER_FLAGS})
 
 if(LMMS_BUILD_WIN32)
 	add_custom_command(

plugins/ZynAddSubFx/CMakeLists.txt

@@ -29,7 +29,7 @@ if(NOT MSVC)
 endif()
 
 IF("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" AND NOT "${CMAKE_CXX_COMPILER_VERSION}" VERSION_LESS "6.0.0")
-	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-misleading-indentation")
+	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-misleading-indentation -Wno-format-truncation")
 ENDIF()
 
 IF(MINGW_PREFIX)