ci: Concurrent Provider Acceptance Tests against Different Terraform Versions

[korenyoni]

What

• Concurrent provider acceptance tests against different terraform versions
• Install Terraform beforehand, do not delegate Terraform SDK v2 to do download and install Terraform

Why

• There is an issue caused when parallel Terraform Provider acceptance tests install Terraform at the same time. Explained fairly well in Remove TF_ACC_TERRAFORM_VERSION from the GHA hashicorp/terraform-provider-kubernetes#2256
• Better testing by using different versions of the Terraform binary (3 latest stable minor versions — their latest stable patch version resolved by terraform-switcher aka tfswitch)

Notes

• Will probably need to test against OpenTofu in the future (as of writing there is no OpenTofu release).

Checklist

• I have read CONTRIBUTING.md.
• I have allowed changes to my fork to be made.
• I have added tests, assuming new tests are warranted.
• I understand that the /test comment will be ignored by the CI trigger unless it is made by a repo admin or collaborator.