Expose GitHub team inputs in inputs.yaml

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Infrastructure as Code Type? (Required)

terraform

### Starter Module? (Required)

terraform - platform_landing_zone

### Use Case (Required)

Many orgs already maintain central GitHub teams for approving production changes. The accelerator currently exposes apply_approvers (usernames/emails) in inputs.yaml and then creates/manages a team for the Apply environment’s required reviewers. There’s no first-class way to reuse an existing org team.

### Proposed Solution (Required)

create_team: false
existing_team_name: platform-approvers

### Important Factoids (Optional)

_No response_

### References (Optional)

_No response_

Start in the terraform - platform_landing_zone module and locate inputs.yaml plus the existing apply_approvers handling. Compare the current managed-team flow with the proposed create_team and existing_team_name inputs. Done means an existing GitHub team can be reused instead of creating or managing a new team, while the current apply_approvers behavior remains clear.

Written by the indexing model from the issue text.