Automatically update provider version in coder/coder upon release

[johnstcn]

Problem

In coder/coder and coder/modules, we reference the coder/coder Terraform provider version in multiple places (https://github.com/coder/coder/pull/16309#pullrequestreview-2580241216)

• provisioner/terraform/testdata
• examples/templates/*
• all of our modules in coder/modules

When a new version of the Terraform provider is released, we currently do not have any way of updating references to the provider version we reference in coder/coder. We currently just "handwave" it by:

1. Not specifying a version at all (most example templates)
2. Specifying >= 0.x (in our modules)
3. Specifying provider version ~> 1.0.0 (2 example templates)

This leads to significant version sprawl and makes it difficult to make informed decisions about what version of the provider is actually in use.

Proposed Solution

          > A better option would be to set version constraints that we auto-bump when new provider versions are released.

We could possibly couple this to the release workflow of the provider:

• After a release, clone the Coder repo at main
• Run a script that updates the provider version in the required places
• Commit and push a branch, and create a PR on coder/coder

Originally posted by @johnstcn in https://github.com/coder/coder/issues/16309#issuecomment-2621308491