fix(float-button): correct border-radius for grouped and standalone buttons

[dariasavinova]

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Fixes #8117

This PR fixes incorrect border-radius in the FloatButton component.

Previously, standalone buttons with shape="square" were affected by :last-child logic, which caused layout issues. This logic (:first-child, :last-child, separators) is now applied only to grouped buttons inside .ant-float-btn-shadow.

Now it works as expected.