Allow set_power_saving even for coex

[bjoernQ]

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Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
• I have used cargo xtask fmt-packages command to ensure that all changed code is formatted correctly.
• My changes were added to the CHANGELOG.md in the proper section.
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Pull Request Details 📖

Description

I think there was a good reason not to allow any other PS than Min for COEX before but it turns out it's fine to set it at least to None and in the case of ESP32-C6 it's almost unusable with any other PS

I changed some (two) examples to set PowerSavingMode::None - could be applied to more / all examples.

We could also now turn coex into a config-option (since it doesn't change the API surface anymore)

This should be the solution for #3049 on ESP32-C6 but unfortunately doesn't help with ESP32 (so not closing it via this)

Testing

Run the wifi_coex example

[jessebraham]

Agree with @bugadani regarding the migration guide, but otherwise LGTM