Adjust conditions for MCM

[mikkamp]

Changes proposed in this Pull Request:

As laid out in the issue woocommerce/woocommerce#53753
The conditions for registering Multi Channel Marketing channels has changed. This PR switches to using a class_exists check instead of a constant.

Detailed test instructions:

1. Test with at least WC 9.5
2. Enable the snippet add_filter( 'woocommerce_gla_enable_mcm', '__return_true' );
3. Onboard with an ads account and make sure at least one campaign is created
4. Go to Marketing > Overview
5. Confirm that the channel is not showing
   
6. Check out this PR and reload the Marketing > Overview page
7. Confirm that the channels and campaigns are showing
   

Changelog entry

• Tweak - Adjust conditions for MCM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.0%. Comparing base (3b936b3) to head (e4e93b1).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ernal/DependencyManagement/CoreServiceProvider.php	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #2739     +/-   ##
===========================================
+ Coverage       66.3%   67.0%   +0.7%     
  Complexity      4693    4693             
===========================================
  Files            816     479    -337     
  Lines          24805   19612   -5193     
  Branches        1273       0   -1273     
===========================================
- Hits           16435   13136   -3299     
+ Misses          8197    6476   -1721     
+ Partials         173       0    -173     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	67.0% <0.0%> (ø)

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Files with missing lines	Coverage Δ
...ernal/DependencyManagement/CoreServiceProvider.php	0.0% <0.0%> (ø)

... and 337 files with indirect coverage changes

[eason9487]

Thanks for the fix! LGTM.

Confirmed that the statuses and campaigns of this plugin can be shown on Marketing > Overview page. I also tested with an older version, WooCommerce 9.0, and the solution is compatible as well.