Update pybind11 submodule to v2.13.6

[dbort]

Summary

Our previous pin @8c7b8dd is a random commit after v2.11.1. There have been 153 commits between that commit and the latest v2.13.6 version.

Test plan

TODO: Will test locally

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7556

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Failures because of deprecated version of actions/download-artifact: v3

❌ 5 New Failures, 1 Cancelled Job

As of commit afe46fd with merge base 7375cf5 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t dae9d6eefeebffee9a77cb5df163f16e0221fb5ec24a93ac2f90a11229db14d4 /exec failed with exit code 127
• pull / test-llama-runner-qnn-linux (fp32, qnn_8a8w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t eceeb6ddcbce55b1d27ee175c254ffa80e00f2ac23a7f7f62c059ac37e5148dc /exec failed with exit code 127
• pull / unittest / linux / linux-job (gh)
  curl: (22) The requested URL returned error:
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t 33d8584087aff2b6bec90dfd3a17fff81404585bb6170839aeb2e6c53f9ea810 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / test-mediatek-models-linux / linux-job (gh)
  ##[error]The operation was canceled.

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[dbort]

#7599 made this same change.