Patch release: 2.4.1

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Planned cherry-picks:

• ng: support path_prefix at router level #4423: ng: support path_prefix at router level (done in 2.4: ng: support path_prefix at router level (#4423) #4549)
• Remove frame-ancestors * CSP directive #4332: Remove frame-ancestors * CSP directive (done in 2.4: Remove frame-ancestors * CSP directive (#4332) #4550)

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Shipped: https://pypi.org/project/tensorboard/2.4.1/