Use Drupal Database API instead of SQL queries in checkPrivateMessageMemberExists method

@uttam Pls check as PHPUnit failing

Working on it, for PHP unit.

Hey,

I have updated the PHP unit method, the PHP unit tests are working fine now, also rebased the 3.0.x branch to this branch as the PHP unit was giving error :
Unable to install modules: module 'private_message' is incompatible with this version of Drupal core.

Now the PHP unit tests are all good but PHP stan gives an error :
Call to an undefined method : Drupal\private_message\Mapper\PrivateMessageMapper::getCanUseRids().

Although the function was used earlier too.
Please have a look.

I have reviewed this issue and found that the checkPrivateMessageMemberExists function was removed as part of this Issue 📌 Refactor private_message_thread_member_widget Active in this Commit under MR-146. Since the function no longer exists, this issue is no longer relevant.

Marking it as outdated and closing it accordingly.