fix: add cleanup for timeout in announce

[smhigley]

Adds cleanup for the timeout set up for announce(), for if the announcer is unmounted while there's an existing timeout.

Ref: https://github.com/microsoft/fluentui/pull/31251#discussion_r1586814428

[codesandbox-ci[bot]]

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[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
FluentProviderWithTheme	virtual-rerender	32	36	10	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	627	644	5000
Button	mount	304	291	5000
Field	mount	1108	1174	5000
FluentProvider	mount	689	719	5000
FluentProviderWithTheme	mount	71	85	10
FluentProviderWithTheme	virtual-rerender	32	36	10	Possible regression
FluentProviderWithTheme	virtual-rerender-with-unmount	74	71	10
MakeStyles	mount	855	846	50000
Persona	mount	1760	1714	5000
SpinButton	mount	1360	1390	5000
SwatchPicker	mount	1570	1485	5000

[fabricteam]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-components react-components: entire library	1.1 MB 267.122 kB	1.1 MB 267.123 kB	21 B 1 B

Unchanged fixtures

| Package & Exports | Size (minified/GZIP) | | ----------------- | -------------------: | | react-accordion
Accordion (including children components) | `100.94 kB`
`30.574 kB` | | react-alert
Alert | `83.737 kB`
`23.475 kB` | | react-avatar
Avatar | `50.175 kB`
`15.944 kB` | | react-avatar
AvatarGroup | `19.702 kB`
`7.794 kB` | | react-avatar
AvatarGroupItem | `64.829 kB`
`20.272 kB` | | react-breadcrumb
@fluentui/react-breadcrumb - package | `116.817 kB`
`32.173 kB` | | react-button
Button | `39.513 kB`
`11.17 kB` | | react-button
CompoundButton | `46.874 kB`
`12.662 kB` | | react-button
MenuButton | `44.292 kB`
`12.544 kB` | | react-button
SplitButton | `52.306 kB`
`14.135 kB` | | react-button
ToggleButton | `56.558 kB`
`13.068 kB` | | react-card
Card - All | `103.992 kB`
`29.35 kB` | | react-card
Card | `97.003 kB`
`27.61 kB` | | react-card
CardFooter | `13.971 kB`
`5.626 kB` | | react-card
CardHeader | `16.214 kB`
`6.386 kB` | | react-card
CardPreview | `14.015 kB`
`5.752 kB` | | react-combobox
Combobox (including child components) | `104.926 kB`
`33.812 kB` | | react-combobox
Dropdown (including child components) | `106.398 kB`
`33.79 kB` | | react-components
react-components: Button, FluentProvider & webLightTheme | `71.104 kB`
`20.52 kB` | | react-components
react-components: Accordion, Button, FluentProvider, Image, Menu, Popover | `221.287 kB`
`62.497 kB` | | react-components
react-components: FluentProvider & webLightTheme | `43.591 kB`
`14.356 kB` | | react-datepicker-compat
DatePicker Compat | `227.451 kB`
`63.531 kB` | | react-dialog
Dialog (including children components) | `116.899 kB`
`36.102 kB` | | react-infobutton
InfoButton | `139.885 kB`
`43.723 kB` | | react-infobutton
InfoLabel | `143.7 kB`
`44.944 kB` | | react-menu
Menu (including children components) | `153.846 kB`
`46.074 kB` | | react-menu
Menu (including selectable components) | `156.532 kB`
`46.623 kB` | | react-message-bar
MessageBar (all components) | `24.204 kB`
`8.983 kB` | | react-persona
Persona | `57.066 kB`
`17.821 kB` | | react-popover
Popover | `128.132 kB`
`40.174 kB` | | react-portal-compat
PortalCompatProvider | `7.944 kB`
`2.588 kB` | | react-table
DataGrid | `168.828 kB`
`46.848 kB` | | react-table
Table (Primitives only) | `45.324 kB`
`14.116 kB` | | react-table
Table as DataGrid | `137.912 kB`
`37.14 kB` | | react-table
Table (Selection only) | `76.328 kB`
`20.545 kB` | | react-table
Table (Sort only) | `74.971 kB`
`20.148 kB` | | react-tag-picker-preview
@fluentui/react-tag-picker-preview - package | `188.273 kB`
`55.749 kB` | | react-tags
InteractionTag | `15.299 kB`
`6.08 kB` | | react-tags
Tag | `29.092 kB`
`9.418 kB` | | react-tags
TagGroup | `82.044 kB`
`24.39 kB` | | react-timepicker-compat
TimePicker | `106.944 kB`
`35.169 kB` | | react-toast
Toast (including Toaster) | `98.667 kB`
`29.793 kB` |

_{🤖 This report was generated against 6eff065e7c743c9e151ef4574dd05b6e75721749}

[fabricteam]

🕵 fluentuiv9 No visual regressions between this PR and main