Reword 'Choose your user name' as 'Choose your account name' in the SSO...

Reword 'Choose your user name' as 'Choose your account name' in the SSO registration template, in order to comply with SIWA guidelines. (#12260)

* Reword as 'Choose your account name'

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>