CI config: update golang version.

[warpfork]

This will become relevant when I bump the go-multihash dependency
in a moment, because that repo now uses golang stdlib features
that are from a relatively modern era.

(Go 1.11 is a long time ago now.)

This is a fairly boring change, so I'm going to merge it immediately as soon as CI confirms it's in fact fine.

[marten-seemann]

#122 fixed all CI failures, so we should be able to enable the Unified CI Setup today.

[warpfork]

... Okay, I meant to extract this as a separate step from #118 just for tidiness, but it turns out that's nontrivial.

Because recent go versions also have stricter compile time checks for... checkptr: converted pointer straddles multiple allocations... it's necessary to bump some other library dependencies at the same time.

I'm going to do go get -u ./... and suspect that this will fix things, and that it will probably be fine.

It will also pull in the multihash v0.15 version, which I was trying to keep "separate", but, I give up.

[warpfork]

#122 fixed all CI failures, so we should be able to enable the Unified CI Setup today.

Oh, super! Let's do that instead.

[marten-seemann]

Done: #124.
We have the same problem there though: https://github.com/ipfs/go-cid/pull/124/checks?check_run_id=2547504037.