Update yoast/wordpress-seo requirement from ^18.9 to ^20.4

Updates the requirements on [yoast/wordpress-seo](https://github.com/Yoast-dist/wordpress-seo) to permit the latest version.
- [Release notes](https://github.com/Yoast-dist/wordpress-seo/releases)
- [Commits](Yoast-dist/wordpress-seo@18.9...20.4)

---
updated-dependencies:
- dependency-name: yoast/wordpress-seo
  dependency-type: direct:production
...

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

composer.json

@@ -12,7 +12,7 @@
     "require": {
         "php": ">=7.1",
         "humanmade/hm-redirects": "~0.7.1",
-        "yoast/wordpress-seo": "^18.9"
+        "yoast/wordpress-seo": "^20.4"
     },
     "autoload": {
         "files": [