Support Ubuntu 26.04 and Qt6

[rhaschke]

• cmake: Inherit Qt version (4/5/6) from rviz
• Enforce cast from char8_t* -> char*
• Enforce using Boost config instead of FindBoost module
• Drop enforcement of CXX_STANDARD
• Fix deprecation warnings

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.93%. Comparing base (1c1194a) to head (19f0d6b).

Files with missing lines	Patch %	Lines
...tion/motion_planning_tasks/src/task_list_model.cpp	0.00%	2 Missing ⚠️
...on/motion_planning_tasks/src/remote_task_model.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #758   +/-   ##
=======================================
  Coverage   52.93%   52.93%           
=======================================
  Files         138      138           
  Lines       10651    10651           
  Branches     1129     1129           
=======================================
  Hits         5637     5637           
  Misses       5006     5006           
  Partials        8        8           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.