workflows: Update stale bot to exclude enhancement label
workflows: Update stale bot to exclude enhancement label

This PR is from comment https://github.com/fluent/fluent-bit/issues/2181#issuecomment-843689076
to explude enhancement label from stable bot.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

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