Switch nav2_voxel_grid to use modern CMake idioms.

[clalancette]

---

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

---

Description of contribution in a few bullet points

This commit does a number of things:

1. Switches to using target_link_libraries in nav2_voxel_grid, which gives us finer-grained control over what dependencies are exported to downstream as public or private. In the particular case of nav2_voxel_grid, this doesn't matter too much, but it will help for other packages.
2. Moves the include directory down one level to include/${PROJECT_NAME}, which is best practice in ROS 2 since Humble.
3. Makes sure to export nav2_voxel_grid as a CMake target, so downstream users of it can use that target.

In my local testing, this change doesn't change the compilation time in any significant way; that is likely because nav2_voxel_grid is such a small package.

Description of documentation updates required from your changes

None needed.

---

Future work that may be required in bullet points

The rest of the packages in this repository need a similar treatment (PRs will be upcoming).

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