Make nav2_bt_navigator use modern CMake idioms.

[clalancette]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Follow-up 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 cleans up nav2_bt_navigator to use modern CMake idioms. In particular:

1. Switch from ament_target_dependencies to target_link_libraries.
2. Make sure to export the target so downstream packages can use it.
3. Move the include directory down one directory, which is 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 follow-up PRs coming to switch more of the packages over to modern CMake 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 4 files with indirect coverage changes