Update dependencies

[jbkempf]

I am quite sure a lot of them are quite outdated :)

[elboulangero]

On that note, I started looking into updating the Go baseline, maybe setting it to 1.18 (available in Ubuntu 22.04, Debian bookworm). And then using the tool modernize to, well, modernize the code accordingly.

I also plan to drop the dependency gopkg.in/tylerb/graceful.v1 at some point after that, as what it does is implemented in the Go stdlib itself.

No ETA.