Add NotExtensible to OngoingStubbing

[TimvdLippe]

This interface is not intended to be subclassed and should therefore be
marked as such.

[codecov-io]

Codecov Report

Merging #1859 into release/3.x will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release/3.x    #1859   +/-   ##
==============================================
  Coverage          86.87%   86.87%           
  Complexity          2518     2518           
==============================================
  Files                314      314           
  Lines               6620     6620           
  Branches             831      831           
==============================================
  Hits                5751     5751           
  Misses               671      671           
  Partials             198      198

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[mockitoguy]

Absolutely!