linux as supported_platform, not specific versions (depreciated) (#747)

-linux only (not specific distros)
-(this functionality is now depreciated by the *prereq_command* functionality in Invoke-AtomicTest)
-reworded section on 'manual tests' after seeing several manually tests today that have been easily automated

Co-authored-by: Carrie Roberts <clr2of8@gmail.com>

atomic_red_team/spec.yaml

@@ -46,8 +46,6 @@ atomic_tests:
   # supported platforms is an array of the OS/platforms this atomic test can be run upon. Values include:
   # - windows
   # - macos
-  # - centos
-  # - ubuntu
   # - linux
   supported_platforms:
     - windows
@@ -125,7 +123,7 @@ atomic_tests:
   #           echo "attack starting"
   #           echo "running command 1: this is the value of the FOOBAR input_argument: #{FOOBAR}"
   #
-  #   - `manual` : a list of manual steps to run. This is most often used when GUI steps are involved that
+  #   - `manual` : a list of manual steps to run. This should only be used when required, for example when GUI steps are involved that
   #              cannot be automated.
   #
   #       Requires the `steps` option that tells the user what to do to invoke the test. This is a