Fix focus order issue in legends overflow callout

[krkshitij]

Previous Behavior

• Pressing the up or down arrow keys while at the first or last legend button in the legends overflow callout causes the page to scroll and collapses the list.
• Pressing the tab or shift-tab keys in the callout doesn't move focus to next or previous interactive element.

New Behavior

• Navigating with the up or down arrow keys, or using the tab or shift-tab keys in the legends overflow callout, cycles focus through legend buttons.

When the legend overflow button is focused, pressing the enter key opens the callout, and pressing the escape key closes it.

[fabricteam]

📊 Bundle size report

✅ No changes found

[codesandbox-ci]

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[fabricteam]

🕵 fluentuiv8 No visual regressions between this PR and main

[AtishayMsft]

Mention in the new behavior that the user has to use the enter and escape keys to open and close the overflow control.