Improve automated PR body with a summary of module version changes

[d-lazenby]

Summary

The PR created by the update-tf-modules workflow currently has a static, uninformative body:

Automated update of Terraform module versions.

This gives reviewers no visibility into what actually changed.

Proposed improvement

Before the create_pull_request step, generate a dynamic PR body that summarises the diff — for example, a Markdown table of module name, old version and new version:

Module	Old version	New version
github.com/org/module	v1.2.3	v1.3.0
registry.terraform.io/...	1.0.0	1.1.0

This could be generated by diffing the changed .tf files with git diff and parsing the source = lines, then passed to the body: input of peter-evans/create-pull-request.

Acceptance criteria

• PR body lists each updated module with its old and new version reference.
• Falls back to the current static message if no structured diff can be parsed.

Relevant file: update-tf-modules.yml (lines 151–157, body: field of create_pull_request step)