Updated name to "Cloud Firestore", updated Firebase app versions

[danwillm]

• The Firebase database the codelab uses is called "Cloud Firestore" or just "Firestore", not "Cloudstore" as it is mentioned to in the README, as well as the codelab

• Firebase versions were out of date - updated from 5.9.3 to 7.13.1

• Made the code that is complete more easily identifiable on the solutions branch

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[danwillm]

@googlebot I signed it!

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