Update main version to 1.6.0

[JacobSzwejbka]

Summary

Update the development version on main from 1.5.0 to 1.6.0 after cutting the release/1.5 branch.

Test plan

• verified version.txt contains 1.6.0
• git diff --check

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22613

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✅ No Failures

As of commit 1243087 with merge base 7dc8641 ():
💚 Looks good so far! There are no failures yet. 💚

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[github-actions]

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