[go/en] update for modern go

[seankhliao]

• [x] I solemnly swear that this is all original content of which I am the original author
• [x] Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
• [x] Pull request touches only one file (or a set of logically related files with similar changes made)
• [x] Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• [x] If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md
  • [x] Yes, I have double-checked quotes and field names!

Updates for modern go:

• new style of build tags
• interface{} -> any
• io/ioutil deprecation
• website migration to go.dev

fixes:

• import alias for "math"
• comment formatting by gofmt