Update module github.com/golang/protobuf to v1.5.3

This MR contains the following updates:

Package	Type	Update	Change
github.com/golang/protobuf	require	patch	v1.5.2 -> v1.5.3

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Release Notes

golang/protobuf

v1.5.3

Compare Source

Notable changes

*( #​1363) jsonpb: accept 'null' as a valid representation of NullValue in unmarshal

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