NcDateTimePicker needs design update for Nextcloud 30

[ChristophWurst]

[susnux]

In words:

• Border width need to use the new two variable (input & input-focused)
• Border color on hover & focus should be --color-main-text and not primary like all other inputs
• The placeholder has a default opacity set, this should be removed as the color should be --color-text-maxcontrast