[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  |
----------------------------+--------------------

Old description:

> There are multiple upgrades since 3.14.0: 3.15.[0-6]
>
> https://github.com/protocolbuffers/protobuf/releases

New description:

 There are multiple upgrades since 3.14.0: 3.15.![0-6]

 https://github.com/protocolbuffers/protobuf/releases

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Comment (by ryandesign):

 The port has no maintainer, but anyone can feel free to submit a (working)
 update in a pull request.

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Ticket URL: <https://trac.macports.org/ticket/62585#comment:3>
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