\Drupal\Core\KeyValueStore\DatabaseStorage::doSetIfNotExists should be protected

Problem/Motivation

The method \Drupal\Core\KeyValueStore\DatabaseStorage::doSetIfNotExists currently has public visibility, but should be protected.

https://git.drupalcode.org/project/drupal/-/blob/11.x/core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php?ref_type=heads#L170

The prefix "do" is typically used for internal implementation, while the regular method that calls "do*" functions acts as a wrapper. For example, the method ::set() is public, while ::doSet() is protected.

By keeping the method ::doSetIfNotExists() public, it forces classes that extend DatabaseStorage to implement it, even if it's not necessary. This method is also not a part of the interface.

Proposed resolution

Change the visibility of the method from public to protected. This change will not break the code that extends DatabaseStorage because in that case, it can change visibility from protected to public but not from public to protected.

The only concern is if someone uses it directly (core doesn't). In that case, we should deprecate its usage and change the signature in Drupal 12. However, I think this is overkill because it is clearly an internal method.