fix(ui5-sidenavigation): menu in collapsed mode width is not longer stretched unnecessary

[dimovpetar]

Background: After making the ui5-responsive-popover a native popover, width: auto stretches it to take the whole available space width.

Solution: Make it take only as much as it needs by leveraging its default width fit-content.

Related to: https://github.com/SAP/ui5-webcomponents/issues/8739