Should remove CMake logic that relies on `lsb_release`

[azeey]

We have some CMake logic that relies on an external tool, lsb_release

gz-transport/CMakeLists.txt

Lines 76 to 90 in d09e1e8

	if (UNIX AND NOT APPLE)
	execute_process(COMMAND lsb_release -cs
	OUTPUT_VARIABLE RELEASE_CODENAME
	RESULT_VARIABLE LSB_RESULT
	OUTPUT_STRIP_TRAILING_WHITESPACE
	)
	if( NOT (${LSB_RESULT} STREQUAL "0"))
	message(WARNING "lsb_release executable not found. Disabling focal-specific workarounds")
	elseif (${RELEASE_CODENAME} STREQUAL "focal")
	set(UBUNTU_FOCAL 1)
	elseif (${RELEASE_CODENAME} STREQUAL "bullseye")
	set(DEBIAN_BULLSEYE 1)
	endif()
	endif()

Building gz-transport on a system where lsb_release is not installed generates a warning (see https://build.ros2.org/job/Jpr__gz_transport_vendor__ubuntu_noble_amd64/3/cmake/). Since we no longer support focal and there's nothing in the code that is conditioned on DEBIAN_BULLSEYE, we should remove this logic in the CMake and any associated macros in the code.