log to stderr instead of stdout

[helvieq499]

Objective

• bevy_log writes logs to stdout (with ANSI formatting), which gets in the way with program output and complicates parsing.
• Closes Logging to stderr #8869

Solution

• Change bevy_log to write to stderr instead of stdout

---

Changelog

Changed:

• Logs write to stderr rather than stdout on desktop targets

Migration Guide

• Capture logs from stderr instead of from stdout
  • Use 2> output.log on the command line to save stderr to a file

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[helvieq499]

I only checked on Windows, but the change is trivial and should work on Linux & macOS.

If there is a better term than "desktop targets" I will edit the changelog to use it instead.

[alice-i-cecile]

This seems like a better solution, and the motivation in the linked issue is good. Let's see if anyone chimes in with an argument not to do this.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[helvieq499]

Would a migration section in the PR be appropriate?
The changes that need to be made as a result of this are outside of the program itself (e.g. piping/redirecting to a file)

[alice-i-cecile]

Yeah, a tiny migration guide would be helpful still to point users in the right direction :)