Changes the Variable 'value' type from string to JSON.

In PrefectHQ/prefect#13543 and associated
changes around May 2024, Variables were updated from simple strings to
JSON objects. The Terraform provider has still been treating them as
strings, so when folks tried to put JSON-compatible values in them,
Terraform would fail to work with them as found in
#254

Related to https://linear.app/prefect/issue/PLA-247/changing-variable-to-a-json-value-in-the-ui-makes-next-terraform-run

Related to #254

The JSON custom type won't work for variables, because they can actually
be almost any type - including standalone numbers and strings.

Goes from a string to a bool in the test to confirm variables can store
multiple types.

Before, when trying to test a variable resource name change, we were
actually creating an entirely new resource because we use the same
(randomized) value for the resource name and the attribute 'name'.

To ensure we're updating the same resource, this hard-codes the resource
name to 'test'. This shouldn't cause any conflicts with other tests
because the resource is created in an ephemeral workspace.