[iOS] Fix for CollectionView with horizontal grid layout has extra space on right end

[Tamilarasan-Paranthaman]

Root Cause of the issue

• The HorizontalItemSpacing was being applied from the GetInsetForSection method, even though it was not a grouped collection

Description of Change

• The HorizontalItemSpacing should not be applied from the GetInsetForSection method when it is not a grouped collection. Therefore, I set the return value to zero, which resolves the issue.

Issues Fixed

Fixes #25433

Tested the behaviour in the following platforms

• [x] Android
• [x] Windows
• [x] iOS
• [x] Mac

Screenshot

• Note: I have set the CollectionView background color to red to make the differences more visually apparent. The red areas in the image represent the spaces.

1. Without Grouping:

Before Issue Fix	After Issue Fix

2. With Grouping

Before Issue Fix	After Issue Fix

[dotnet-policy-service[bot]]

Hey there @Tamilarasan-Paranthaman! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jsuarezruiz]

/azp run

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[jsuarezruiz]

/azp run

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