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>

.eslintrc.js

@@ -51,8 +51,9 @@ module.exports = {
         destructuring: 'all',
       },
     ],
+    'unicorn/prefer-string-slice': 'error',
   },
-  plugins: ['babel', '@emotion', 'cypress'],
+  plugins: ['babel', '@emotion', 'cypress', 'unicorn'],
   settings: {
     react: {
       version: 'detect',