Bump macOS deployment target to 12.0

[jathu]

Summary

We want to enable MPS AOT for macOS. This requires a minimum target deployment of 12.0. Example error:

2025-05-30T01:57:11.3334050Z /Users/runner/work/executorch/executorch/pytorch/executorch/backends/apple/mps/runtime/MPSGraphBuilder.mm:174:1: error: 'MPSGraphExecutable' is only available on macOS 12.0 or newer [-Werror,-Wunguarded-availability-new]
2025-05-30T01:57:11.3334100Z MPSGraphExecutable*
2025-05-30T01:57:11.3334140Z ^

Given 12.0 is already at end of life, are there any other problems with this increase?

Test plan

rg "10(\.|_)15" --hidden -g '!**/{third-party,.git}'

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11247

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❌ 1 New Failure

As of commit 0c4e95f with merge base b02ac1b ():

NEW FAILURE - The following job has failed:

• trunk / test-qnn-model (fp32, ic3) / linux-job (gh)
  RuntimeError: Command docker exec -t eabc5410f4303d4633e83bc29fd45bf19a2d8257cd9355c825b775c88b0f0395 /exec failed with exit code 1

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[shoumikhin]

Mind checking which version we use internally for Buck?
We can already remove some "@availabe(macOS 11" checks.
And potentially we may bump it even to 12.3 and remove some more of the @available checks.

[jathu]

I create a task to clean up checks below 12.0: #11259