Improve version compatibility

[ammario]

In https://github.com/coder/coder/discussions/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 (https://github.com/coder/terraform-provider-coder/issues/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: https://github.com/coder/terraform-provider-coder/blob/3cc9802d8c2ac8a6254b1c7242fbc9e19756bc46/provider/workspace.go#L83-L90

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