Incorporate new build tag format in Go 1.17

[qnighy]

Go 1.17 introduced a new format for build tags, namely //go:build. This PR incorporates the change so that the build tag is written in both formats. This is how go fmt updates the existing code.

Go versions 1.16 and earlier used a different syntax for build constraints, with a "// +build" prefix. The gofmt command will add an equivalent //go:build constraint when encountering the older syntax.

https://pkg.go.dev/cmd/go#hdr-Build_constraints

[qnighy]

@izumin5210 could you look at this?

[qnighy]

@izumin5210 We have been extensively using izumin5210/gex and izumin5210/grapi and want to have these packages up-to-date with the current state of the ecosystem. You know we have currently submitted three: this PR, #71, and #72.

We are willing to continue contributing the patches if needed, but we cannot merge them ourselves. We need your help.

I understand you are busy and probably aren't as interested in it as before anymore. If you cannot afford time or other resources to review them, we are prepared to propose these alternatives:

• to inherit the modules' ownerships and help maintain them, if you are willing to give us ones
• or to fork them ourselves.

I'd appreciate it if I could hear your thoughts on it. Thank you in advance!