GPL custom validator: Use tl_expected from libexpected-dev instead (backport #2212)

[mergify]

Needs PickNikRobotics/generate_parameter_library#322

and reverts #2127

---

This is an automatic backport of pull request #2212 done by Mergify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (3d1f948) to head (d849803).
⚠️ Report is 3 commits behind head on jazzy.

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #2239   +/-   ##
=======================================
  Coverage   83.99%   83.99%           
=======================================
  Files         153      153           
  Lines       14629    14629           
  Branches     1302     1302           
=======================================
  Hits        12287    12287           
  Misses       1866     1866           
  Partials      476      476           

Flag	Coverage Δ
unittests	83.99% <ø> (ø)

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Files with missing lines	Coverage Δ
..._trajectory_controller/validate_jtc_parameters.hpp	79.16% <ø> (ø)

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