[cloud_firestore] Update autoId generation to match native implementation more closely

[KevinTheGray]

Description

Currently the flutter implementation of Firestore generates a new document ID in the same way it is geenrated for the RTDB. However, this behavior has been modified for Firestore, and it no longer generates IDs in the same way. I was motivated to make this change because:

1. The plugins should behave as close as possible to the native implementation. It was very obvious to me when I saw the keys that were being generated that they were not mimicking expected behavior.

2. It's probably negligible, but the new code is a lot less complex and under less constraints.

Related Issues

flutter/flutter#15715

flutter/flutter#15715 is resolved because it now matches how the Android/iOS generate document IDs. That is, from the pool of characters from String "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", we choose 20 random ones and generate an ID.

Checklist

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• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[kroikie]

@KevinTheGray Thanks for this PR, looks good to me, once the tests pass I will merge.

[kroikie]

LGTM

[collinjackson]

Suggested change

	* Updated how document Ids are generated to more closely match native implementations.
	* Updated how document ids are generated to more closely match native implementations.