Notes: Remove specific update information for legacy -> WebExtension

[Croydon]

Once support for Firefox 60 ESR is dropped this specific information in the notes (and potential other places) can be replaced/removed for good.

People who will update at a later time (which means they have used for at least half a year an insecure browser.......) will need to figure the changes out by themselves.

This means:

• removing all changelog summarys (0.9.0 - 0.14.0) except the very current one (0.15.0)
• update wording in notes header ("... all add-ons were required to be rewritten...")