cleanup: avoid CMake warning-message markup (#6265)

CMake `message(WARNING "message to display")` uses a simple markup
language, where newlines in non-indented text are presumed to
delineate paragraphs that should be wrapped and separated. Avoid
that by adding a little indentation, so the text renders as a
single block.

Author: devbww

super/ExternalProjectHelper.cmake

@@ -20,10 +20,10 @@ include(GNUInstallDirs)
 message(
     WARNING
         [==[
-Super builds are no longer recommended, and will be removed on 2022-05-01
-or shortly after.  If you want to automatically download and build the
-`google-cloud-cpp` dependencies we recommend that you use `vcpkg` as
-described in /doc/contributor/howto-guide-setup-cmake-environment.md
+  Super builds are no longer recommended, and will be removed on 2022-05-01
+  or shortly after.  If you want to automatically download and build the
+  `google-cloud-cpp` dependencies we recommend that you use `vcpkg` as
+  described in /doc/contributor/howto-guide-setup-cmake-environment.md
 ]==])
 
 set(GOOGLE_CLOUD_CPP_EXTERNAL_PREFIX