ci(tmux): Add tmux 3.4 to test matrix

[tony]

See also:

• https://github.com/tmux/tmux/blob/3.4/CHANGES
• ci(tmux): Add tmux 3.4 to test matrix libtmux#524

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b35f743) 70.38% compared to head (87e32e1) 70.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
- Coverage   70.38%   70.33%   -0.06%     
==========================================
  Files          26       26              
  Lines        1955     1955              
  Branches      409      400       -9     
==========================================
- Hits         1376     1375       -1     
- Misses        453      455       +2     
+ Partials      126      125       -1     

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