Fix TAN List Description

In https://git.scc.kit.edu/reg-app/reg-app/-/blob/branch-2.7/bwreg-webapp/src/main/resources/msg/messages_en.properties#L1031

the description mentioned "five" TANs. But the Number of OTP TANs ist adjustable on LinOTP site. So a more generic description would be more accurate:

twofa_backup_tan_list_values = Here you see the next values of your TAN list. These values must be used in the displayed order.