Update image minimum size restrictions from 600x600 to 400x400

[JorgeMucientes]

Closes: #11367

Description

Reduce the minimum ad image size restriction from 600x600 to 400x400px

The API will now handle upscaling the image when needed if the uploaded image is smaller than 600x600px but larger than 400x400px.

The goal is to unblock users when creating Blaze ads, that have smaller product images. Full context here: p1713450773231259/1712831081.385859-slack-C069RG07W6S

Testing instructions

1. Start Blaze campaign creation flow
2. Click on Edit ad button
3. Attempt adding an image under 400x400px and check dialog is displayed showing invalid image message
4. Attempt to add an image over 400x400px and check is added correctly

Images/gif

imageAtLeast400x400.mp4

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 18.4. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Flavor	Jalapeno
	Build Type	Debug
	Commit	d84e06f
	Direct Download	woocommerce-prototype-build-pr11368-d84e06f.apk

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 41.14%. Comparing base (98cc713) to head (d84e06f).
Report is 8 commits behind head on trunk.

Files	Patch %	Lines
...reation/ad/BlazeCampaignCreationEditAdViewModel.kt	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk   #11368   +/-   ##
=========================================
  Coverage     41.14%   41.14%           
  Complexity     5218     5218           
=========================================
  Files          1058     1058           
  Lines         62316    62316           
  Branches       8506     8506           
=========================================
  Hits          25640    25640           
  Misses        34372    34372           
  Partials       2304     2304           

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[0nko]

LGTM