Use latest Nginx version, 1.29.6, in System Tests

[xlamorlette-datadog]

Following the delivery of Nginx Datadog 1.13.0, use, in System Tests, the latest Nginx version (1.29.6).

This PR works with DataDog/system-tests#6473.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.85%. Comparing base (b9e382b) to head (765020f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #325   +/-   ##
=======================================
  Coverage   68.85%   68.85%           
=======================================
  Files          56       56           
  Lines        7471     7471           
  Branches     1058     1058           
=======================================
  Hits         5144     5144           
  Misses       1820     1820           
  Partials      507      507           

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