DEV: Drop diffhtml dependency (#27695)

We switched to morphlex via:

- c5f1fc3a08

- bc561eb332

.github/dependabot.yml

@@ -64,7 +64,6 @@ updates:
       - dependency-name: "@json-editor/json-editor"
       - dependency-name: "chart.js"
       - dependency-name: "chartjs-plugin-datalabels"
-      - dependency-name: "diffhtml"
       - dependency-name: "magnific-popup"
       - dependency-name: "pikaday"
       - dependency-name: "moment"