You need to define a rewrite map which can only be done in server/vhost config files, not in htaccess files. You'll need to add something like:
RewriteMap lc int:tolower
Then in your htaccess file, you can create a rule like:
RewriteCond %{REQUEST_URI} [A-Z]
RewriteRule ^(.*.aspx)$ ${lc:$1} [L,NC]
This will check that there are capital letters in the URI, then apply the map which turns everything into lowercase letters.
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