Update Cortex-M backend status from proof of concept to beta

[rascani]

As discussed in 4/1/26 partner sync.

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 6ffc8c6 with merge base 6823b33 ():

NEW FAILURE - The following job has failed:

• Build Presets / linux (pybind, linux.arm64.2xlarge, executorch-ubuntu-22.04-gcc11-aarch64) / build (gh)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[mergennachin]

For Beta, we start adhering to public API deprecation and lifecycle policy

https://docs.pytorch.org/executorch/main/api-life-cycle.html

[rascani]

Updated to remove the comment about APIs changing, and added some details about how it has been tested so far.

@AdrianLundell - let me know if you're okay with this. I'll wait on merging until you're on board.