Reduce Duplication in TLSUtils and Connect.go Packages.

#### Feature Description

[`./tlsutils/generate.go`](https://github.com/hashicorp/consul/blob/master/tlsutil/generate.go) shares code with [`agent/connect/generate.go`](https://github.com/hashicorp/consul/blob/master/agent/connect/generate.go) where they are both doing similar things. The Connect generate.go file says that the connect based ParseSigner should be the function used, rather than TLSUtils. This ticket is to track if [this comment](https://github.com/hashicorp/consul/blob/master/agent/connect/generate.go#L97-L98).

Potential solution:
TLSUtils holds all TLS based functions. Connect references the TLSUtils and we remove this comment.
Deprecate TLSUtils and move all cert logic to Connect generation. Update references from TLSUtils to point to connect methods.

This arose from #7623 and #8898 changes

#### Use Case(s)

Clean Code.

Compare tlsutil/generate.go with agent/connect/generate.go, starting at the Connect ParseSigner comment referenced in the issue. Determine which of the two proposed ownership directions is intended before changing references. Done means the duplicated TLS certificate logic and outdated comment are resolved consistently across both packages.

Written by the indexing model from the issue text.