Rename document_name to document_key

[schmidt-sebastian]

Use consistent naming in IndexState and IndexEntries. This is not a breaking change if merged by Feb 1 as the schema went live in the same release: #3226

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-firestore

  Type	Base (66797a8)	Merge (ebb56f0)	Diff
  aar	1.24 MB	1.24 MB	-3 B (-0.0%)
  apk (release)	3.33 MB	3.33 MB	-28 B (-0.0%)

Test Logs

• Base (66797a8): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-binary-size/1487921991724830722
• Merge (ebb56f0): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/3383/binary-size/1488199560797884416

Notes

• Commit (ebb56f0) is created by Prow via merging PR base commit (66797a8) and head commit (390da0b).

1. https://storage.googleapis.com/firebase-sdk-metric-reports/UWi7zxakuS.html

[google-oss-bot]

@schmidt-sebastian: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
smoke-tests	390da0b	link	/test smoke-tests

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