tmux: Add tmux 3.6 to testgrid

[tony]

Changes

Bump tmux max version to tmux 3.6

See release notes: https://github.com/tmux/tmux/blob/3.6/CHANGES#L1

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46.21%. Comparing base (4601185) to head (e921282).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/libtmux/common.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #607   +/-   ##
=======================================
  Coverage   46.21%   46.21%           
=======================================
  Files          19       19           
  Lines        1876     1876           
  Branches      294      294           
=======================================
  Hits          867      867           
  Misses        894      894           
  Partials      115      115           

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