[terra-data-grid] Resize handle communication fix

[cm9361]

Summary

What was changed:
The resize handle was updated so that its ARIA label was not communicated when a non-header cell is focused.

Why it was changed:
The resize handle was being communicated when a non-header cell was focused.

Testing

This change was tested using:

• WDIO
• Jest
• Visual testing (please attach a screenshot or recording)
• Other (please describe below)
• No tests are needed

Reviews

In addition to engineering reviews, this PR needs:

• UX review
• Accessibility review
• Functional review

Additional Details

N/A

This PR resolves:

UXPLATFORM-9803

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[sdadn]

Now that #1851 is merged, I think these changes should be done on the table subcomponents.

[chrismichalewicz]

@cm9361 I tested this both for Functional and A11y, since the issue requires JAWS to test, with the scope strictly to just the issue you are addressing.

Tested with Edge + JAWS

Keyboard navigation:
Navigated the column headers with arrow keys,
• Resize handles only were communicated when they have focus
• Column headers when in focus communicated without mention of the resize handle
• Navigating to a non-header cell is communicated appropriately

Mouse:
• Unable to recreate the issue, selecting a non header cell does not communicate the resize handle as it previously was.
• Noticed with a mouse that the new column widths aren't communicated, but this is also occuring on the Prod doc site so is not related to this work. Will follow up outside this PR on this issue.

cc: @mjpalazzo