Automate bumping of default cpd_version using Renovate

[jor2]

Description:

Currently, the cpd_version variable in variables.tf is set manually to a specific version (default: 5.1.1). To ensure the module always uses the latest supported Cloud Pak for Data 5.x.x version, we should automate this update using Renovate, similar to how we manage module versions.
(NOTE: its also used in some of the submodules)

Tasks:

• Configure Renovate to detect and update the cpd_version variable in Terraform files.
• Ensure Renovate PRs are created when a new Cloud Pak for Data 5.x.x version is released.
• Add or update Renovate configuration as needed.
• Document the automation in the repo.

References:

IBM Cloud Pak for Data versions
Renovate Terraform documentation

Acceptance Criteria:

• Renovate automatically creates PRs to update the cpd_version variable when new versions are available.
• PRs include change log or release notes if possible.
• Manual intervention is minimised, and the process is consistent with module version updates.

[ocofaigh]

• Also need to manage cloud_pak_deployer_release version (in root and submodule)
• Also need to manage value of cloud_pak_deployer_image (using image sha)