comparing uint to uint.

[vimanyu]

Fixes compiler errors after github ubuntu runners were upgraded to 20 from 18 and gcc from 10.1.0 to 10.2.0.

build/external/src/firestore/Firestore/core/src/local/leveldb_persistence.cc:199:36: error: comparison of integer expressions of different signedness: ‘uint64_t’ {aka ‘long unsigned int’} and ‘long int’ [-Werror=sign-compare]
2021-03-01T23:04:48.9071111Z   199 |     if (count < old_count || count > std::numeric_limits<int64_t>::max()) {
2021-03-01T23:04:48.9071795Z       |                              ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2021-03-01T23:04:48.9072445Z cc1plus: all warnings being treated as errors

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[vimanyu]

Fix is already being pushed in another PR.
#7639

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Coverage Report

Affected SDKs

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  SDK overall coverage changed from 88.68% (2f21a7f) to 88.67% (fe85ff2) by -0.00%.

  Filename	Base (2f21a7f)	Head (fe85ff2)	Diff
  leveldb_key.cc	96.96%	96.48%	-0.48%
  task.cc	95.21%	96.41%	+1.20%

Test Logs

• Base (2f21a7f9): https://github.com/firebase/firebase-ios-sdk/actions/runs/615237965
• Head (fe85ff28): https://github.com/firebase/firebase-ios-sdk/actions/runs/615300481