fix: date fields should have same placeholders throughout admin interface

[DevinWalker]

Bug Report

User Story

As an admin I want a consistent experience throughout the admin interface when working with Give.

** WP-Admin > Donations > Donations **

This field should have "Start date" and "End date" as field placeholders.

Current Behavior

Right now the admin interface has different placeholders throughout the admin interface.

Bug Type

• I am not sure whether this functionality ever worked as expected.

Acceptance Criteria

• I see consistent Start Date and End Date placeholders on all the date fields throughout the plugin.
• As a developer I've checked that all placeholders are the same throughout the plugin.
• All date pickers respect the date format selected under WP-Admin > Settings > General.