<IfModule mod_rewrite.c>
    Options +FollowSymLinks -MultiViews

    RewriteEngine On

    RewriteCond %{REQUEST_FILENAME} -d
    RewriteRule ^(.*)/$ $1.html

    RewriteCond %{REQUEST_FILENAME}.html -f
    RewriteRule !.*\.html$ %{REQUEST_FILENAME}.html [L]

    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteCond %{REQUEST_FILENAME}\.html -f
    RewriteRule ^(.*)$ $1.html [NC,L]

    # Protect some contents
    RewriteRule ^.*/?\.git+ - [F,L]

    RewriteCond %{ENV:HTTPS} !on
    RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
</IfModule>

Source: https://github.com/vercel/next.js/discussions/10522#discussioncomment-3687225