Update nav2_behaviors to use modern CMake idioms.

[clalancette]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Followup to #4357
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

This PR changes nav2_behaviors to use modern CMake idioms. In particular:

1. Get rid of ament_target_dependencies() in favor of target_link_libraries()
2. Make sure to export the target so downstream consumers can use it.
3. Relegate rclcpp_action to a test_dependency, since it is only needed for tests.
4. Move the include directory down one level in include/${PROJECT_NAME}, which is the ROS 2 best practice since Humble.

Description of documentation updates required from your changes

None needed.

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Future work that may be required in bullet points

There will be additional PRs upcoming to switch more of the Navigation2 codebase over to these idioms.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 2 files with indirect coverage changes