go: use last major version to bootstrap current version

[kevinburke1]

There is nothing special about Go 1.7, and a more recent version of Go
is both supported and should compile more quickly - on my machine this
saves about 5 seconds. See discussion in #71370 for more on why make
this change.

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Closes #71370

[kevinburke1]

This doesn't affect the ongoing effort to update the Go formula to 1.16.

[Bo98]

Go pull requests take a very long time (24 hours) to go through CI.

Since we've agreed that we should use the same version on ARM and Intel, and we are going to update to 1.16 stable bootstrap on ARM anyway (1.15 doesn't work on ARM) as a part of the Go 1.16 update, I will handle this in that Go 1.16 pull request when I rebase it - it's much more efficient on our limited resources to do both at the same time. I hope to get Go 1.16 merged this weekend.

Thanks for the contribution though!