Upgrade to Go 1.18

[alvinlin123]

According to Go's 1.18 release notes there are a lot of goodies, so we should upgrade.

Sooner or later, there will be dependencies we want to use that requires Go 1.18 (Hugo is an example).

So we should upgrade.

When we upgrade to Go 1.18, would be a good idea to upgrade Hugo to latest version too because i was not able to do so in https://github.com/cortexproject/cortex/pull/4814