[BUG]Block updation of knn.enabled flag.

[harishbhakuni]

What is the bug?
Today, user can update knn.enabled flag even after an index is created which should not be allowed. Users can close the index, update this setting and open the index back which is not an expected behaviour. We have seen some issues due to this recently as well during version upgrades to 2.17.
Also, user can today restore an existing index with knn.enabled setting as true as an index with knn.enabled setting as false. we need to block this setting from getting updated during snapshot restore as well.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Create a kNN enabled index.
2. Close the index.
3. Open the index.

What is the expected behavior?
knn.enabled setting should be final and should not be allowed to get updated.

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
This index setting should be made final, so that it cannot be updated even after closing the index.
Also, during snapshot restore, there is a list of index settings for which we do not support updation. We should add this setting in that list.