Update the release-notes (#13612)

Update the release-notes (#13612)

* Removed the "Tools have been updated for Vaadin 7.2 with the following changes:"
* Removed the security fix notice for 7.1.11
* Removed the text about using the experimental Eclipse plug-in
* Specify that 7.2 is a minor release
* Clarified the known issues section
* Listed Windows Phone 8 in supported devices
* Replaced vaadin.com/pro-account with vaadin.com/pro-tools and vaadin.com/support

Change-Id: I8e03b65453419c9cb9f4570067675156f2be76fe