releng: use mylyn 3.21 to build help plug-ins

releng: use mylyn 3.21 to build help plug-ins

The current build process is aligned with the APIs of this
version. Trace Compass only depends on mylyn for the
generation of the help plug-ins and not at runtime.

Change-Id: If0e796f7b96710207c5cfa395e6dba49f99421e3
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/96114
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI