Remove Ubuntu 18.04 from supported distributions

Ubuntu 18.04 is now in "Extended Security Maintenance", but otherwise
gets no updates. Remove it from supported distributions (regarding
installing packages); users can of course still use it.

Users are encouraged to upgrade to Ubuntu 20+.

CMakeLists.txt

@@ -180,22 +180,6 @@ add_boolean_option(SANITIZE_ADDRESS False "enable the address sanitizer (ASan)"
 if (SANITIZE_ADDRESS)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address -fno-omit-frame-pointer -fno-common")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -fno-omit-frame-pointer -fno-common")
-  # There seems to be some incompatibility with pthread_create and the RT scheduler, which
-  # results in pthread_create hanging.
-  #
-  # When we switch from Ubuntu 16.04 to 18.04, we found that running with the address sanitizer,
-  # the pthread_create function calls were not working. The inital thought was that we were
-  # trying to create a thread that was not-blocking and would eventually crash the machine during
-  # the run. After more debugging, we found that we would never even start the thread. We narrowed
-  # down the first two instances of pthread_create in the gNB and NR UE to be sctp_eNB_task and
-  # one_thread, respectively. We found that adding sleeps, and various other pauses to the threads
-  # had not effect. From there, we found that if we add an abort(); prior to the thread loop, we
-  # do not execute that. This indicated to us that the problem is not likely to be a non-blocking
-  # thread, but perhaps and issue with pthread_create itself. From there we begain to research the
-  # issue on the web. See: https://github.com/google/sanitizers/issues/1125
-  #
-  # Google searching indicates this appears to be a problem since at least 2018. This could be something
-  # wrong in the pthread library, or something subtly wrong in this CMakeLists.txt. Use Ubuntu 20.04 instead.
 endif ()
 
 add_boolean_option(SANITIZE_THREAD False "enable the address sanitizer (TSan)" ON)