Consolidate "variable not declared" errors from -var-file

[apparentlymart]

In order to give better feedback to users who have made typos in their variable declarations or definitions, we introduced an error, later demoted to a warning after discussion in #19424, if a .tfvars file includes a definition for a variable that hasn't been declared. We're now recommending the use of environment variables to set "ambient" variable values that made available to all Terraform configurations in a session, since that warning does not apply to those.

However, existing uses with large .tfvars files currently lead to a lot of warnings, because we generate one warning per undeclared variable, and the result is therefore quite overwhelming and redundant. Instead, we should gather all of the undeclared variables into a single list and report a single warning that explains that these should be migrated to use environment variables, using similar text to what we currently have in each individual message.

[ghost]

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