chore: update tofu version for test matrix

[lechnerc77]

Purpose

• Update OpenTofu version matrix to include latest release 1.9.*
• Update wrong configuration of tofu regression test (tofu_version)

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[X] Other... Please describe: Test setup

How to Test

• Test the code via automated test

go test ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

Other Information

Release notes for OpenTofu 1.9.0: https://github.com/opentofu/opentofu/releases/tag/v1.9.0

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[lechnerc77]

Same issue with test fixtures as for Terraform see: https://github.com/SAP/terraform-provider-btp/actions/runs/12707001344/job/35421070143

Should be merged once this is fixed see #973