What is this squawk 7777 target at an altitude of 62,700' over Germany?
This is a military test transponder, not a real aircraft. The callsign GAFTT08 stands for German Air Force Test Transponder 08. When you zoom in on the map, you can see the reconstructed position move in a pattern around the fixed true position:


(adsbexchange.com)

As far as I can tell, the military does not publicly say what exactly they are doing here. The only sources I could find are from forums:


  GAFTTxx = German Air Force Test Transponder


(flugzeugforum.de)


  German Air Force Test Transponder. So not an aircraft, not a drone. Don’t know why it is online all the time, but it is. Sometimes over Würzburg, sometimes over Ingolstadt.


(forum.scramble.nl)

Additionally, the squawk code of 7777 is used in Germany for fixed test transponders according to Wikipedia (thanks to ymb1 for pointing this out):


  7777 US, Germany, Belgium, Netherlands: Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE)


(List of transponder codes - Wikipedia)

This is also confirmed by the following ICAO definition:


  Code 7700 is reserved for recognizing an aircraft
  in emergency. (Codes 7711 to 7717 and 7721 to 7727 are
  reserved for SAR operations and code 7777 for monitoring
  the ground transponder.)


(ICAO - SSR CODE ASSIGNMENT SYSTEM, emphasis mine)