Update drewm/mailchimp-api requirement from 2.4 to 2.5.4 (#15)

Updates the requirements on [drewm/mailchimp-api](https://github.com/drewm/mailchimp-api) to permit the latest version.
- [Release notes](https://github.com/drewm/mailchimp-api/releases)
- [Commits](drewm/mailchimp-api@v2.4...v2.5.4)

---
updated-dependencies:
- dependency-name: drewm/mailchimp-api
  dependency-type: direct:production
...

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composer.json

@@ -27,7 +27,7 @@
   },
   "require": {
     "php": "~8.0",
-    "drewm/mailchimp-api": "2.4",
+    "drewm/mailchimp-api": "2.5.4",
     "dwnload/wp-settings-api": "^3.3",
     "egulias/email-validator": "~3.2",
     "thefrosty/wp-utilities": "^3.2",