fix(NotificationsPanel): fixed glitchy animation after closing the panel

[devadula-nandan]

Contributes to #4651

Applied fade-out animation with "forwards" fill mode to prevent animation reset upon completion, resolving the issue.

What did you change?

packages/ibm-products/src/components/NotificationsPanel/NotificationsPanel.js

How did you test and verify your work?

storybook

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