ServerSignature Off
won't work on custom error pages which is what you have in the default XAMPP installation.
This from \extra\httpd-default.conf
#
# Optionally add a line containing the server version and virtual host
# name to server-generated pages (internal error documents, FTP directory
# listings, mod_status and mod_info output etc., but not CGI generated
# documents or custom error documents).
# Set to "EMail" to also include a mailto: link to the ServerAdmin.
# Set to one of: On | Off | EMail
#
ServerSignature On
The error messages are contained within xampp\apache\error directory and are in various languages.
You can turn custom error messages off in the httpd.conf file by commenting out this line:
# Multi-language error messages
Include conf/extra/httpd-multilang-errordoc.conf
To:
# Multi-language error messages
# Include conf/extra/httpd-multilang-errordoc.conf
or comment out the entries in the conf file it refers to.
You should now be able to control the Apache default error messages using your .htaccess file although I have not tested your .htaccess file to see if it all works.