refactor: use eslint.config.mjs instead of .eslintrc

This was done automatically by running
`npx  @eslint/migrate-config .eslintrc.js`

Signed-off-by: Philip Molares <philip.molares@udo.edu>

package.json

@@ -137,6 +137,8 @@
     "url": "https://github.com/hedgedoc/hedgedoc.git"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.2.0",
+    "@eslint/js": "^9.19.0",
     "@hedgedoc/codemirror-5": "5.65.12",
     "abcjs": "6.4.4",
     "babel-cli": "6.26.0",
@@ -166,6 +168,7 @@
     "flowchart.js": "1.18.0",
     "fork-awesome": "1.2.0",
     "gist-embed": "2.6.0",
+    "globals": "^15.14.0",
     "highlight.js": "10.7.3",
     "html-webpack-plugin": "4.5.2",
     "imports-loader": "1.2.0",