Assuming you are using Apache the following lines in .htaccess file under the /contact folder will do the trick:
RewriteEngine On
RewriteBase /
RewriteRule ^(.*)/index.html$ %{CONTEXT_PREFIX}/folder/$1/ [R=301,L]
RewriteRule ^index.html$ %{CONTEXT_PREFIX}/folder/ [R=301,L]
where folder is contact in your example.
Earlier I suggested using the following rule for a shortcut of the above two:
RewriteRule ^(.*)index.html$ %{CONTEXT_PREFIX}/folder/$1 [R=301,L]
However, this has an unwanted side-effect of redirecting say something like https://www.example.com/contact/myindex.html to https://www.example.com/contact/my
Note: Please be careful not to cause redirection "loops" like:
xxx/index.html -> xxx/ -> xxx/index.html -> xxx/ ...