npm: bump @x-govuk/govuk-eleventy-plugin from 6.2.1 to 6.4.0

Bumps [@x-govuk/govuk-eleventy-plugin](https://github.com/x-govuk/govuk-eleventy-plugin) from 6.2.1 to 6.4.0.
- [Release notes](https://github.com/x-govuk/govuk-eleventy-plugin/releases)
- [Commits](x-govuk/govuk-eleventy-plugin@v6.2.1...v6.4.0)

---
updated-dependencies:
- dependency-name: "@x-govuk/govuk-eleventy-plugin"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>