Add scan-classes mixin when adding Api4 classes

[colemanw]

This used to be a soft-requirement but is now strongly encouraged.

• See avoid duplication in Api4 LegacyEntityScanner civicrm/civicrm-core#33371
• See Add upgrade to add scan-classes mixin for extensions with Api4 classes #412

[colemanw]

FYI @ufundo

[colemanw]

@totten test failure appears unrelated to this pr:

Fatal error: Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 8.0.0". You are running 7.4.29. in /home/homer/buildkit/build/warmup/web/sites/all/modules/civicrm/vendor/composer/platform_check.php on line 24

[totten]

@colemanw I think that notice should be fixed, but I think it's a red-herring. AFAICS, it only appeared once. And it's talking about the "cache warmup" which prepopulates some stuff in composer's cache. (But if warmup fails... then composer will just download the necessary files on-demand.)

Looking at more failures, the common theme (near the end) is:

Enabling extension "org.example.civixsnapshot"

In MySearch.php line 7:
                                                         
  Class "CRM_Contact_Form_Search_Custom_Base" not found  
                                                         

ext:enable [-r|--refresh] [--ignore-missing] [--] [<key-or-name>...]

I think this reveals a latent problem:

• The make-snapshots job tries several scenarios for generating extensions... including the kitchensink scenario. That calls every generator, including generate:search.
• If you call generate:search, then you make a class which extends CRM_Contact_Form_Search_Custom_Base and therefore (in modern times) depends on legacycustomsearches. But the info.xml doesn't declare the dependency.
• When you enable class-scanning, then it will try to read that class-file during installations. But the class cannot be read because legacycustomsearches is disabled.

I'll throw up a PR to see if it's easy to drop generate:search. (If it's hard to remove, then we can also fix it -- with a silver-lining of having an example of auto-updating the <requires> directive.)

[totten]

civibot, test this please