Update GridItem naming in DxcGrid component

[GomezIvann]

Checklist

• [x] Build process is done without errors. All tests pass in the /lib directory.
• [x] Self-reviewed the code before submitting.
• [x] Meets accessibility standards.
• [x] Added/updated documentation to /website as needed.
• [x] Added/updated tests as needed.

Purpose The result of the poll done by the team was to rename the DxcGrid.GridItem to just DxcGrid.Item.

Description Rename the DxcGrid.GridItem compound component to DxcGrid.Item. This change is applied at the library level.