Revert react to version 18

In the realease notes at https://docusaurus.io/blog/releases/3.7 , they
mention that they are adding early support for react 19 in preparation
for docusaurus 4. They will keep supporting react 18 in 3.x series. By
the time it becomes required, the rest of the ecosystem will catch up to
react 19.

Author: nsrahmad