Update module golang/protobuf to v1.5.2

This MR contains the following updates:

Package	Type	Update	Change
github.com/golang/protobuf	require	patch	v1.5.1 -> v1.5.2

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Release Notes

golang/protobuf

v1.5.2

Compare Source

Notable changes:

• (#​1306) all: deprecate the module
• (#​1300) jsonpb: restore previous behavior for handling nulls and JSONPBUnmarshaler

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