Bump simple-icons from 13.21.0 to 15.18.0

Bumps [simple-icons](https://github.com/simple-icons/simple-icons) from 13.21.0 to 15.18.0.
- [Release notes](https://github.com/simple-icons/simple-icons/releases)
- [Commits](simple-icons/simple-icons@13.21.0...15.18.0)

---
updated-dependencies:
- dependency-name: simple-icons
  dependency-version: 15.18.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

package.json

@@ -16,7 +16,7 @@
     "grunt-contrib-copy": "^1.0.0",
     "grunt-contrib-sass": "^2.0.0",
     "grunt-wp-readme-to-markdown": "~2.1.0",
-    "simple-icons": "^13.0.0"
+    "simple-icons": "^15.18.0"
   },
   "license": "MIT",
   "bugs": {