[FEATURE] Add autofocus attribute to BE login

Adding the HTML5 autofocus attribute to the
backend login box's username field.

Resolves: #49228
Releases: 6.2
Change-Id: I5a00d5ec260ade81fb83553a50358cddf95eaebb
Reviewed-on: https://review.typo3.org/21500
Reviewed-by: Eric Chavaillaz
Tested-by: Eric Chavaillaz
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe