Bump govuk-markdown from 0.3.0 to 0.7.0

Bumps [govuk-markdown](https://github.com/x-govuk/govuk-markdown) from 0.3.0 to 0.7.0.
- [Release notes](https://github.com/x-govuk/govuk-markdown/releases)
- [Commits](x-govuk/govuk-markdown@v0.3.0...v0.7.0)

---
updated-dependencies:
- dependency-name: govuk-markdown
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -36,7 +36,7 @@
     "govuk_template_jinja": "^0.24.1",
     "govuk-elements-sass": "^3.1.3",
     "govuk-frontend": "^4.6.0",
-    "govuk-markdown": "^0.3.0",
+    "govuk-markdown": "^0.7.0",
     "gulp": "^4.0.0",
     "gulp-nodemon": "^2.5.0",
     "gulp-sass": "^5.0.0",