Update Firestore example to use @for and AsyncPipe #2068

Job	Run time
Build	42s
Contribute Node 18 (ubuntu-latest)	1m 16s
Contribute Node 18 (macos-latest)	2m 19s
Contribute Node 18 (windows-latest)	4m 56s
Test chrome-headless	1m 13s
Test Node 18 (ubuntu-latest)	36s
Test firefox-headless	1m 10s
Test Node 20 (ubuntu-latest)	25s
Test Node 18 (macos-latest)	44s
Test Node 20 (macos-latest)	38s
Test Node 18 (windows-latest)	1m 17s
Test Node 20 (windows-latest)	1m 18s
Publish (NPM)	0s
Branch protection	1s
	16m 35s

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