(dev/core#4074) Make CRM_Core_BAO_CMSUser CMS agnostic

[herbdool]

Fixes https://lab.civicrm.org/dev/core/-/issues/4074. Splits up the logic into each CMS in the userSystem. This functionality is used when using a profile to create a user account.

I've tested it with creating a user account and also checking the username which happens via ajax on the profile (on Drupal 9). Both still work.

[civibot]

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[herbdool]

jenkins retest this please

[civicrm-builder]

Can one of the admins verify this patch?

[herbdool]

Turns out I've got a Backdrop-related bug to fix as well, which would benefit from this being merged first and built on top. (I may have to make it a separate PR and rework it again if/when this PR gets merged).

Any interest in reviewing? @demeritcowboy @eileenmcnaughton

[demeritcowboy]

Sure I can take a look just probably won't be until the weekend.

[herbdool]

Thank you @demeritcowboy!

[herbdool]

@demeritcowboy This is my follow-up to this one: #25371 (or could be done first if this one gets stuck).

[herbdool]

Oops. I accidentally closed this PR. I'll see if I can update/reopen it or I'll create a replacement.

[demeritcowboy]

Ah git. The old state of the tree should still be at cd46160, e.g. https://github.com/civicrm/civicrm-core/tree/cd46160, if needed.

[herbdool]

@demeritcowboy thank you. Tests are now running again.

[eileenmcnaughton]

@demeritcowboy this has passed now - it looks like your review has been addressed?

[demeritcowboy]

I didn't fully review it the first time and haven't re-reviewed this yet after being combined with the other PR.

[demeritcowboy]

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