build: bump golang: 1.17 -> 1.18

No change to go.mod's `go` stanza, so no changes in code compatibility.

However, it's used for building our docker images and release
binaries, and for fuzz testing in our nightly workflow.

Some test-related changes with the dns lookup built-in function's
error handling; and the hardcoded signature. Running

    go test ./topdown -run TestTopdownJWTEncodeSignECWithSeedReturnsSameSignature -count 10000

makes me believe that for whatever reason the signature changed,
it's at least stable.

topdown/http_test: Test-only change to accomodate this change in Go (https://go.dev/doc/go1.18):

    Certificate.Verify now uses platform APIs to verify certificate
    validity on macOS and iOS when it is called with a nil
    VerifyOpts.Roots or when using the root pool returned from
    SystemCertPool.

We're keeping the old message for go <= 1.17; in a silly-simple way.

Also:

* ci: build and test two old golang version on macos|linux

  We'll drop golang 1.15, keep one unsupported version (1.16).

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>