[TASK] Increase input placeholder contrast ratio

Set the color of placeholders to #767676 to meet WCAG AA
conformance for background-color #fefefe.

Resolves: #92621
Releases: master
Change-Id: I573876390cc628793817a5498479b0389afd05cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66262


Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>