Point to 4.21 content in welcome page.

Change-Id: I7cd6948f9e07b6918a605f7d40e69cc454dee927
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform/+/182809
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>