ioutil + jwt deprecations

[kfcampbell]

I know #111 exists but I figured it'd be worth proposing this PR without the google/go-github bump in case it's worth taking by itself.

The Go standard library's ioutil package is deprecated. This updates those usages to either io or os package functions.

It also updates a test-only usage of the jwt.StandardClaims to jwt.RegisteredClaims.

BREAKING CHANGE: Since these io and os package functions were introduced in Go 1.16, this PR updates the go.mod file's go line to version 1.16.

[jamestelfer]

All these methods were introduced in Go 1.16 -- should go.mod be advanced to match this?

[kfcampbell]

Great catch @jamestelfer! I've done so, and marked the PR as a breaking change in the writeup above.

[wlynch]

LGTM!

One thing I want to double check w/ @bradleyfalzon before submitting - how do you feel about bumping go directives? Technically this is a breaking change for older Go versions, but most people are unlikely to hit (upstream only supports last 2 versions, so as of now >= 1.21). Do you think this warrants a major version bump?
I'm okay with having minor releases track the oldest upstream supported version - wdyt?

[bradleyfalzon]

Yeah, this would be breaking the build for some users, but wouldn’t require a code change. The remediation for the user is increasing the version of go which shouldn’t be a breaking change whilst we’re just increasing minor version change.

So I think it’s fine to not be a major version change, especially as mentioned, we’re only requiring a less ancient release.