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>

dev-test/backends/test/index.html

@@ -183,7 +183,7 @@
           h('h2', {}, 'Related Post'),
           h('h3', {}, post.get('title')),
           h('img', { src: post.get('image') }),
-          h('p', {}, post.get('body', '').substr(0, 100) + '...'),
+          h('p', {}, post.get('body', '').slice(0, 100) + '...'),
         ) : null;
       }
     });