Minor changes

• improve calendar selection border by making an after element to not adjust cell spacing, and remove borders between selected days in contiguous ranges (PR #19211 by lorejoh12)

Patches

• Fix Pivot's isPivotItem check for IE11 compatibility (PR #19238 by behowell)
• Fix website and legacy demo in IE 11 (PR #19237 by ecraig12345)
• Bump @fluentui/example-data to v8.2.6 (PR #18566 by behowell)