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

[MacPorts]

#62585: protobuf3-cpp: update to 3.15.6
----------------------+---------------------------
 Reporter:  Javabien  |      Owner:  (none)
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:            |       Port:  protobuf3-cpp
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 There are multiple upgrades since 3.14.0:
 3.15.0
 3.15.1
 3.15.2
 3.15.3
 3.15.4
 3.15.5
 3.15.6

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