chore: prep opentofu support

[lechnerc77]

Purpose

This PR adds the prerequisites for supporting OpenTofu from a testing perspective namely:

• Adding OpenTofu into the devcontainer setup
• Adding the test of the latest provider against a test matrix of OpenTofu versions
• Adding a schema regression test of the provider against the latest version of the OpenTofu CLI
• Removal of the OpenTofu compatibility check as the new workflows already test this functionality

All tests are executed on demand and not linked to any other event like PRs

Merging this PR does NOT mean that we officially support OpenTofu.

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: testing setup for OpenTofu

How to Test

• Execute the GH Action workflows
• Rebuild the devcontainers

What to Check

Verify that the following are valid:

• The GH Action workflows get executed successfully
• The devcontainers get build successfully and the OpenTofu CLI is available in version 1.8.1

Other Information

n/a

Checklist for reviewer

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

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