pin gem version

[JC-DSIT]

• pin the gem temporarily to investigate why 6.5.0 won't pull in components

Proposed changes

What changed

since the release of tech_docs_gem 6.5.0 new sites throw an error when importing govuk components. This seems fine for existing sites

Checklist

Before you request approval you should confirm that:

• the pull request has a clear title with a short description about the update in the documentation
• GitHub actions all pass
• you have tested the changes with a fresh build against the latest version of the tech-docs-gem
• you have linked this PR to an issue (or explained why none exists)
• you have updated documentation if needed