ioutil.ReadAll is deprecated since Go1.16 - see https://tip.golang.org/doc/go1.16#ioutil
VictoriaMetrics requires at least Go1.18, so it is OK to switch from ioutil.ReadAll to io.ReadAll.
This is a follow-up for 02ca2342ab
Previously ScrapeConfig.clone() was improperly copying promauth.Secret fields -
their contents was replaced with `<secret>` value.
This led to inability to use passwords and secrets in `-promscrape.config` file.
The bug has been introduced in v1.77.0 in the commit 67b10896d2
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2551