Releases for Date time day

• Fix Issue #3286909 by Project Update Bot, Andriy Khomych: Automated Drupal 10 compatibility fixes
• Fix Issue #3354186 by Rachinskiy, Andriy Khomych: Deprecated function: DateTime::createFromFormat(): Passing null to parameter #2 ($datetime) of type string is deprecated in Drupal\Component\Datetime\DateTimePlus::createFromFormat() (line 251 of core/lib/Drupal/Component/Datetime/DateTimePlus.php

• Adds minimal PHP 7.2 version.
• PHP stan and sniffer code updates.
• Adds github actions.
• Fix tests and removes deprecated code.

• Wrong start and endtime when editing
• Field datetime handling isn't in accordance with core

• Some code refactoring and small adjustments for the base widget to be able to work with seconds storage field option
• README updates
• Issue #2998094: Fix start time is not saved
• Issue #3065642: Fix end time can be earlier than the start time
• Issue #3138885: Drupal 9 readiness

Bug fixes and updates in README.

• Fixed some common typos and comment from security review
• The website encountered an unexpected error. Error: Call to undefined method buildDateWithIsoAttribute

• Updated view formatter.
• Fixed labels.
• Restructured widgets.
• Added Travis builder.
• Added base kernel tests.