go 1.16

[chenrui333]

• ci: use go1.16
• deps: use go1.16 (go modules by default)
• doc: update go version ref

[chenrui333]

let me know what you folks think on this PR, cc @InTheCloudDan @yusinto

[vlymar]

Thanks @chenrui333! Have you had a chance to test ld-find-code-refs built with go 1.16? Go tends to be good at backwards compatibility but we'll want to sanity check things. We'll try to get to testing and merging this shortly.

[InTheCloudDan]

I tested locally after last merge and this builds

[chenrui333]

I have maintained bunch of homebrew go formulae, I did not observe any go1.16 build issues. And thanks @InTheCloudDan for testing it out. :)