Use rclcpp::Executor instead of rclcpp::executor::Executor(deprecated)

[bmagyar]

In response to warning: ‘using Executor = class rclcpp::Executor’ is deprecated: use rclcpp::Executor instead [-Wdeprecated-declarations]

[codecov-commenter]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   40.72%   40.72%           
=======================================
  Files          24       24           
  Lines         550      550           
  Branches      279      279           
=======================================
  Hits          224      224           
  Misses         98       98           
  Partials      228      228           

Flag	Coverage Δ
#unittests	40.72% <100.00%> (ø)

Impacted Files	Coverage Δ
.../include/controller_manager/controller_manager.hpp	66.66% <ø> (ø)
controller_manager/src/controller_manager.cpp	56.89% <ø> (ø)
...ontroller_manager/test/test_controller_manager.cpp	26.92% <ø> (ø)
controller_manager/test/test_load_controller.cpp	20.27% <ø> (ø)
...er_parameter_server/test/test_parameter_server.cpp	15.90% <100.00%> (ø)

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