Please check the release notes where we made a security fix to the htaccess file:
Removing that entire block is really a very bad idea for security reasons.
You could compare the differences between your pre-upgrade htaccess with the one in the release and focus specifically on that part - the fix for subfolders was provided in the issue here:
If you cannot figure out the changes you need to make to support your current Mautic setup in a subfolder (eg the regex to use) then maybe consider hiring someone to help with that - it should be a relatively simple job.