Update main version to 1.5.0

[JacobSzwejbka]

Bumps the main-branch version from 1.4.0a0 to 1.5.0 following the 1.4 release branch cut.

Test plan:

• git diff --check
• python setup.py --version in full package mode
• EXECUTORCH_BUILD_MINIMAL=1 python setup.py --version

Both package modes report 1.5.0 with the current commit suffix.

Authored with Codex.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21564

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