Use fixed thresholds for Trinary yaml (backport #5278)

[mergify]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	-
Primary OS tested on	Ubuntu
Robotic platform tested on	gazebo simulation , Jazzy
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

Supersedes #5138

This PR uses fixed values for the threshold saved to the yaml file, in Trinary mode. Similarly to ROS 1.
With this change, Trinary maps loaded created and loaded with the map_server, will now load correctly. This is because the pixel values saved in this mode are constants, so when loading the map the thresholds must lie between the pixel values for correct loading. The thresholds supplied by the user are still used for thresholding the input map before saving (to reduce noise)

Description of documentation updates required from your changes

None

Description of how this change was tested

• Use ros2 run nav2_map_server map_saver_cli with various arguments. The threshold saved should be fixed to 0.65 and 0.196 in Trinary mode. Other modes should not be affected.

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Future work that may be required in bullet points

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

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This is an automatic backport of pull request #5278 done by [Mergify](https://mergify.com).

[mergify]

@mergify[bot], all pull requests must be targeted towards the main development branch.
Once merged into main, it is possible to backport to @jazzy, but it must be in main
to have these changes reflected into new distributions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
nav2_map_server/src/map_io.cpp	92.51% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

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