chore(lint): enable gosec (#7792)

Enable "gosec" linter.

Exclude:

- All G115 (integer overflow) findings, to be fixed separately.

Add targeted gosec annotations for:

- non-crypto math/rand usage
- md5 used only for file change detection
- G114 ("net/http serve with no timeout settings"), to be fixed
  separately.

Other findings fixed.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/reload/setup.go

@@ -60,7 +60,7 @@ func setup(c *caddy.Controller) error {
 		j = i / 2
 	}
 
-	jitter := time.Duration(rand.Int63n(j.Nanoseconds()) - (j.Nanoseconds() / 2))
+	jitter := time.Duration(rand.Int63n(j.Nanoseconds()) - (j.Nanoseconds() / 2)) // #nosec G404 -- non-cryptographic jitter.
 	i = i + jitter
 
 	// prepare info for next onInstanceStartup event