Drop support for versions of Go prior to 1.15.0

[axw]

Closes #655

[apmmachine]

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• Start Time: 2022-01-13T03:16:18.399+0000

• Duration: 33 min 19 sec

• Commit: 2d0a8ce

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Skipped	174
Total	7345

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[marclop]

Thanks!

[axw]

It's a little up in the air as to what the minimum supported version for v2 will be. I'm going to merge this as it is, and we'll nail that down before releasing v2.0.0.