Migrate hardware_interface's on_init method

[urfeex]

The preferred signature of hardware_interface::on_init has been changed upstream. This change avoids a deprecation warning on that.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 5.11%. Comparing base (1b121b7) to head (d8a9484).
⚠️ Report is 475 commits behind head on main.

Files with missing lines	Patch %	Lines
ur_robot_driver/src/hardware_interface.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1464      +/-   ##
========================================
+ Coverage   3.59%   5.11%   +1.52%     
========================================
  Files         13      33      +20     
  Lines        947    3404    +2457     
  Branches     152     427     +275     
========================================
+ Hits          34     174     +140     
- Misses       843    3228    +2385     
+ Partials      70       2      -68     

Flag	Coverage Δ
unittests	5.11% <0.00%> (+1.52%)	⬆️

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