chore: move tabindex to the root slot of Spinner

[tomi-msft]

Fixes #31025

[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
FluentProviderWithTheme	virtual-rerender	33	36	10	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	625	645	5000
Button	mount	293	306	5000
Field	mount	1126	1162	5000
FluentProvider	mount	709	702	5000
FluentProviderWithTheme	mount	80	82	10
FluentProviderWithTheme	virtual-rerender	33	36	10	Possible regression
FluentProviderWithTheme	virtual-rerender-with-unmount	74	73	10
MakeStyles	mount	848	852	50000
Persona	mount	1766	1720	5000
SpinButton	mount	1384	1381	5000

[codesandbox-ci]

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[fabricteam]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-spinner Spinner	24.71 kB 8.35 kB	24.673 kB 8.341 kB	-37 B -9 B

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
react-components react-components: Button, FluentProvider & webLightTheme	71.104 kB 20.52 kB
react-components react-components: Accordion, Button, FluentProvider, Image, Menu, Popover	221.141 kB 62.432 kB
react-components react-components: FluentProvider & webLightTheme	43.591 kB 14.356 kB
react-portal-compat PortalCompatProvider	7.944 kB 2.588 kB

_{🤖 This report was generated against 6441cd2b0654e1b1404c5c3bcea59c7c66d9617a}

[fabricteam]

🕵 fluentuiv9 No visual regressions between this PR and main

[smhigley]

Just the one comment, otherwise looks good!