CPS-332: Style fix for discount apply button
Created by: deb1990
Overview
In https://github.com/civicrm/civicrm-core/pull/18410/files#diff-6b993747c85ceec4b537726331c934f4aeac58a54152f850dab5aec2942a1009L65, the button markup was changed from input
to button
. This PR applies the same to Cividiscount.