Drupal 10 compatability

HI @jonathanshaw

The refactoring of the module is a bit more complex than I thought. This module has a lot of dependencies on other modules, especially for tests. Currently, xero_sync requires "drupal/xero": "^2.0@alpha" which doesn't have the support of D10. I noticed that the xero module has 3.0.x-dev version with D10. Although drupal/commerce_xero (has already supported D10) requires drupal/xero:2.0.alpha (D8 and D9 only). It is messed.

Also, I've tried to refactor the deprecated matcher at() from PHPUnit. It didn't work properly without full SynchronizerTest refactoring. I created a custom copy of this matcher in the module for internal usage as a temporary solution.

I guess need to resolve issues with these module's dependencies first. Without that, the Drupal CI tests can't execute cause the composer failed.

Added core_version_requirement: ^9 || ^10 to the submodule.
Replaced $this->prophesize() with new Prophet())->prophesize(). see https://www.drupal.org/node/3365413 →

Merged, thanks

Automatically closed - issue fixed for 2 weeks with no activity.