chore: fix deprecated ioutil readall

[idnandre]

Description

After Go 1.16, function ioutil.ReadAll is deprecated and may be removed from future versions of Go. Go recommend to change ioutil.ReadAll into io.ReadAll. Also, in another file like this https://github.com/hashicorp/vault/blob/main/api/auth/approle/approle.go#L175 it already use io.ReadAll. So this PR will change deprecated function and make all code use io.ReadAll instead of ioutil.ReadAll.

Ref : https://pkg.go.dev/io/[email protected]#ReadAll

TODO only if you're a HashiCorp employee

• [ ] Labels: If this PR is the CE portion of an ENT change, and that ENT change is getting backported to N-2, use the new style backport/ent/x.x.x+ent labels instead of the old style backport/x.x.x labels.
• [ ] Labels: If this PR is a CE only change, it can only be backported to N, so use the normal backport/x.x.x label (there should be only 1).
• [ ] ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature of a public function, even if that change is in a CE file, double check that applying the patch for this PR to the ENT repo and running tests doesn't break any tests. Sometimes ENT only tests rely on public functions in CE files.
• [ ] Jira: If this change has an associated Jira, it's referenced either in the PR description, commit message, or branch name.
• [ ] RFC: If this change has an associated RFC, please link it in the description.
• [ ] ENT PR: If this change has an associated ENT PR, please link it in the description. Also, make sure the changelog is in this PR, not in your ENT PR.