[BUGFIX] Source element within picture element has wrong src attribute

Change it from src to srcset as described in the latest HTML5
specification for master and 6.2

Resolves: #63107
Releases: master, 6.2
Change-Id: Ia3180873af948db30bb60f2f071e5ccfe2283baf
Reviewed-on: http://review.typo3.org/34405
Reviewed-by: Martin Eberle <martin.eberle@gmail.com>
Tested-by: Martin Eberle <martin.eberle@gmail.com>
Reviewed-by: Martin Ficzel <martin.ficzel@gmx.de>
Reviewed-by: Ingo Schmitt <is@marketing-factory.de>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Jens Jacobsen <typo3@jens-jacobsen.de>
Tested-by: Jens Jacobsen <typo3@jens-jacobsen.de>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>