Improve version compatibility

[ammario]

In coder/coder#13122 a user sees empty values when referencing ${data.coder_workspace.me.template_id} in their template. This happens because their coder server version is behind the provider, and this kind of issue will continue to appear as we implement more provider functionality (#220, #219).

There are two things we should do to fix this:

1. In our docs under each attribute and resource, declare the Coder version requirements

2. Replace code like:
   

   terraform-provider-coder/provider/workspace.go

   Lines 83 to 90 in 3cc9802

   	templateID := os.Getenv("CODER_WORKSPACE_TEMPLATE_ID")
   	_ = rd.Set("template_id", templateID)
   	templateName := os.Getenv("CODER_WORKSPACE_TEMPLATE_NAME")
   	_ = rd.Set("template_name", templateName)
   	templateVersion := os.Getenv("CODER_WORKSPACE_TEMPLATE_VERSION")
   	_ = rd.Set("template_version", templateVersion)

with LookupEnv and fail the build when a value is missing instead of using blanks.

[johnstcn]

As an extra check, we could pass in the Coder version to the provisioner, and then check the version against the oldest version of Coder supported by the provisioner. WDYT?

[ammario]

As an extra check, we could pass in the Coder version to the provisioner, and then check the version against the oldest version of Coder supported by the provisioner. WDYT?

I think passing the version is a good idea, but I think we should add rules to convert old parameters into new ones instead of "breaking" at certain version boundaries.

We want to minimize upgrade anxiety caused by breakage.