VED-874: Enable Terraform state locking.

[mfjarvis]

Summary

• Routine Change
• 🤖 Operational or Infrastructure Change

Prevent simultaneous Terraform deployments for the same workspace. See:
https://developer.hashicorp.com/terraform/language/state/locking
https://developer.hashicorp.com/terraform/language/backend/s3#use_lockfile-1.

Reviews Required

• Dev
• Test
• Tech Author
• Product Owner

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all or part of the acceptance criteria of the ticket, and the code is in a mergeable state.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• I have ensured the changelog has been updated by the submitter, if necessary.

[github-actions]

This branch is working on a ticket in the NHS England VED JIRA Project. Here's a handy link to the ticket:

VED-874

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud