Switch nav2_theta_star_planner to 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

Change nav2_theta_star_planner to modern CMake idioms:

1. Switch from ament_target_dependencies to target_link_libraries
2. Export a target so downstream packages can use it.
3. Push the include directory down a level, 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

This is part of a larger series updating Navigation2 packages to modern CMake idioms. There will be follow-up PRs to convert more of the packages.

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 ✅

Files	Coverage Δ
nav2_theta_star_planner/src/theta_star.cpp	97.95% <ø> (ø)
nav2_theta_star_planner/src/theta_star_planner.cpp	92.91% <ø> (ø)