chore: replace deprecated String.prototype.substr() (#6333)

* chore: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

* refactor: add prefer slice lint rule and fix errors

Co-authored-by: erezrokah <erezrokah@users.noreply.github.com>

packages/netlify-cms-widget-file/src/withFileControl.js

@@ -319,6 +319,7 @@ export default function withFileControl({ forImage } = {}) {
       if (!value || value.length <= size) {
         return value;
       }
+      // eslint-disable-next-line unicorn/prefer-string-slice
       const text = `${value.substring(0, size / 2)}\u2026${value.substring(
         value.length - size / 2 + 1,
         value.length,