CI: disable test against GAP 4.11

[fingolfin]

@gaehler here you go

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.94%. Comparing base (787b273) to head (087e111).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   78.94%   78.94%           
=======================================
  Files           3        3           
  Lines         741      741           
=======================================
  Hits          585      585           
  Misses        156      156