Replace namespaces robot_state and robot_model with moveit::core

[ayushgargdroid]

Description

Fixes issue #1878.
Using moveit::core namespace instead of robot_state and robot_model namespaces.

These namespaces acted as aliases for backward compatibility defined here.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov-io]

Codecov Report

Merging #1924 into master will decrease coverage by 0.02%.
The diff coverage is 56%.

@@            Coverage Diff             @@
##           master    #1924      +/-   ##
==========================================
- Coverage   50.28%   50.26%   -0.03%     
==========================================
  Files         313      313              
  Lines       24647    24647              
==========================================
- Hits        12394    12389       -5     
- Misses      12253    12258       +5

Impacted Files	Coverage Δ
...moveit/move_group_interface/move_group_interface.h	36.84% <ø> (ø)	⬆️
...it/collision_distance_field/collision_env_hybrid.h	0% <ø> (ø)	⬆️
...e/moveit/robot_interaction/kinematic_options_map.h	0% <ø> (ø)	⬆️
...parameterization/model_based_state_space_factory.h	100% <ø> (ø)	⬆️
...n_distance_field/collision_common_distance_field.h	93.33% <ø> (ø)	⬆️
...eit/planning_scene_monitor/current_state_monitor.h	0% <ø> (ø)	⬆️
...rc/default_capabilities/move_action_capability.cpp	38.88% <ø> (ø)	⬆️
...ude/moveit/robot_interaction/interaction_handler.h	0% <ø> (ø)	⬆️
...g_scene_interface/src/planning_scene_interface.cpp	1.23% <ø> (ø)	⬆️
...eit/constraint_samplers/union_constraint_sampler.h	12.5% <ø> (ø)	⬆️
... and 127 more

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