[date-time-picker] Replace custom date-picker and time-picker extensions with original components

[web-padawan]

Describe your motivation

Currently, vaadin-date-time-picker uses two extensions of vaadin-date-picker and vaadin-time-picker element.
This introduces complexity for native CSS styling, because users need to keep in mind these different tag names.

Note, actual extensions don't override any functionality (at least in the current version) and are just basic classes:

Describe the solution you'd like

1. Remove custom extensions and instead change the component to use vaadin-date-picker and vaadin-time-picker;
2. Update themes to replace ThemableMixin styles with selectors like ::part(input-field) to apply rounded borders:

web-components/packages/date-time-picker/theme/lumo/vaadin-date-time-picker-styles.js

Lines 25 to 28 in b35879e

	[part~='input-field'] {
	border-top-right-radius: 0;
	border-bottom-right-radius: 0;
	}

web-components/packages/date-time-picker/theme/lumo/vaadin-date-time-picker-styles.js

Lines 43 to 46 in b35879e

	[part~='input-field'] {
	border-top-left-radius: 0;
	border-bottom-left-radius: 0;
	}

This only applies to Lumo, and in Material theme we don't have any CSS customizations for these picker extensions.

Additional context

This hasn't been added to the original V24 styling breaking changes, but IMO there is still time to include this change.

[web-padawan]

Looks like we can't use ::part() after ::slotted() - see w3c/csswg-drafts#5161 and w3c/csswg-drafts#3896.
However, we can still implement this border radius thing using SlotStylesMixin and custom CSS properties.