[TASK] Move login.html to EXT: backend

Move templates/login.html to EXT: backend.

Change-Id: I6b7813f8f4d05256ea1fa8a12c54ba2e660202cc
Fixes: #47873
Releases: 6.2
Reviewed-on: https://review.typo3.org/20464
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring