[MacPorts] #62585: protobuf3-cpp: update to 3.15.6

[MacPorts]

#62585: protobuf3-cpp: update to 3.15.6
----------------------------+--------------------
  Reporter:  Javabien       |      Owner:  (none)
      Type:  update         |     Status:  new
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:
Resolution:                 |   Keywords:
      Port:  protobuf3-cpp  |
----------------------------+--------------------
Changes (by mf2k):

 * type:  defect => update


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Ticket URL: <https://trac.macports.org/ticket/62585#comment:2>
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