Update version to 3.7.0

[TeBoring]

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[ghost]

BTW, did we append the -rc2 suffix by hands?

[TeBoring]

@acozzette How did you do that?
I think we have to do it manually currently. Different languages use different style.

[ghost]

@acozzette How did you do that?
I think we have to do it manually currently. Different languages use different style.

https://github.com/protocolbuffers/protobuf/blob/master/update_version.py handles each language separately. I wasn't sure the style for each language before, but I can use this as an example.

[acozzette]

I ran Hao's script to update the version to 3.8.0, and then I went through and replaced 3.8.0 with 3.7.0-rc2. I had to do it that way because the script doesn't know how to do rc releases yet, but I wanted to run it anyway to make sure I didn't miss any places that needed to be updated.

[TeBoring]

@haon4 SG.

[ghost]

#5750 to support the rc suffix in update_version.