chore: set packed = false on field_behavior extension (#7034)

PiperOrigin-RevId: 604675854
Source-Link: googleapis/googleapis@42c04fe
Source-Link: https://github.com/googleapis/googleapis-gen/commit/a1ed8a97a00d02fe456f6ebd4160c5b2b000ad75
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