GitVersion 5 getting old, update to GitVersion 6.3.0

[rmunn]

I recently ran into problems running the Chorus unit test suite on Linux, as GitVersion 5.10.3 complained that it couldn't find any usable version of libssl. (It was looking for a version that was too old, and my Linux install didn't have that version installed). Updating to GitVersion 6.3.0 allowed me to run the Chorus tests on Linux, and it seems like a good idea in general to keep our dependencies up-to-date.

This will require a couple changes in GitVersion.xml: the tag: property has been renamed to label: so it's less ambiguous with Git tags, and the prevent-increment-of-merged-branch-version property has been split: prevent-increment-of-merged-branch-version: false should now become:

prevent-increment:
  of-merged-branch: false