Avoid t() calls, instead use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t()

Problem/Motivation

After running coder for Drupal Best Practices, I found the following issues regarding t() calls:

phpcs --standard=DrupalPractice .

FILE: /optional_end_date/src/Plugin/Field/FieldType/OptionalEndDateDateRangeItem.php
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FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
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 52 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
 73 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
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Proposed resolution

Need to replace t() calls with $this->t() and use \Drupal\Core\StringTranslation\StringTranslationTrait wherever required.