GridLayout: Validate that all panel IDs are unique

Description

GridLayout which is used by GlDashboardLayout requires all dashboard panels in the config to have unique IDs, this is documented in Update dashboard layout documentation (!4794 - merged) and the code comments, however it's not enforced.

A problem caused by this is that when panel IDs are not unique it doesn't generate an error, yet the grid fails to render all items correctly - see the screenshot below. This means that this typebug would be difficult to discover and fix in production because it's not obvious.

Screenshot

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Checklist

Make sure the following are completed before closing the issue:

1. Assign the correct component label to this issue.
2. Create an MR with the additions or updates needed.
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5. When applicable, create an MR in Pajamas to update the component status.
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7. When applicable, create an issue in Pajamas using the "Figma update" issue template to update the Figma UI Kit. Ping a designer on your team for awareness and bring the issue to your team planning meeting for prioritization and scheduling.
8. When introducing a major or breaking change, communicate the changes within the Engineering Week in Review.
9. 🎉 Congrats, you made it! You can now close this issue.