Upgrade to golangci-lint 1.61.0

[brandur]

I accidentally upgraded my golangci-lint locally, so figured I may as
well take it up a version in CI as well.

The main new drama queen for this version is fatcontext [1]. It looks
like it's a well-intentioned lint, which tries to avoid having callers
set context in loops and inadvertently create a super deep context that
causes performance issues:

func notOk() {
        ctx := context.Background()

        for i := 0; i < 10; i++ {
                ctx = context.WithValue(ctx, "key", i) // "nested context in loop"
                _ = ctx
        }
}

Unfortunately, as with many well-intentioned golangci-lint additions,
its heuristic is overly broad, and it produces a lot of false positives
(in fact, 100% of lines it found were false positives). I've marked
lines nolint:fatcontext for now, but this may be one that we disable
if it gets too annoying because it's almost certainly more cost than
benefit.

The good news is that we get upgraded version of gosec, and some of
the lines that were marked as nolint and which were false positives
on unsafe integer conversion (they were always safe) can be removed.

[1] https://github.com/Crocmagnon/fatcontext