Kinship report: change max number of generations.

[SNoiraud]

Fixes #12004

[codecov-commenter]

Codecov Report

Merging #1236 (f7235f5) into master (9c29b8e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1236   +/-   ##
=======================================
  Coverage   41.16%   41.16%           
=======================================
  Files        1062     1062           
  Lines      144703   144703           
=======================================
  Hits        59562    59562           
  Misses      85141    85141           

Impacted Files	Coverage Δ
gramps/plugins/textreport/kinshipreport.py	92.30% <100.00%> (ø)

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