api: drop dependency on go-rootcerts

[dolmen]

The github.com/hashicorp/go-rootcerts module doesn't seem necessary anymore with recent Go versions:

• The package was initially created to workaround a Darwin-only bug that has been fixed in Go 4 years ago
• I filled a question about the relevance of that module (hashicorp/go-rootcerts#21) almost one year ago, and nobody replied. Obviously nobody is maintaining that module.
• Dropping this dependency will also remove a dependency on github.com/mitchellh/go-homedir (see also hashicorp/go-rootcerts#22 about another way to drop that dependency)

$ cd api
$ go mod graph | grep go-rootcerts          
github.com/hashicorp/vault/api github.com/hashicorp/go-rootcerts@v1.0.2
github.com/hashicorp/go-rootcerts@v1.0.2 github.com/mitchellh/go-homedir@v1.1.0
github.com/hashicorp/vault/api@v1.0.5-0.20200519221902-385fac77e20f github.com/hashicorp/go-rootcerts@v1.0.1
github.com/hashicorp/go-rootcerts@v1.0.1 github.com/mitchellh/go-homedir@v1.1.0
$ go mod graph | grep go-homedir  
github.com/hashicorp/go-rootcerts@v1.0.2 github.com/mitchellh/go-homedir@v1.1.0
github.com/hashicorp/go-rootcerts@v1.0.1 github.com/mitchellh/go-homedir@v1.1.0 

[dolmen]

There are only a few references to rootcerts.Config in this repo:

• api/client.go
• api/ssh_agent.go
• builtin/credential/cert/backend_test.go

If I write the patch myself, is there a chance for it to be accepted?