chore: migrate firestore to java-firestore

[BenWhitehead]

Code has been migrated to https://github.com/googleapis/java-firestore

[schmidt-sebastian]

LGTM after resolving conflicts. Please make sure to migrate the newest version of the code.

[codecov]

Codecov Report

Merging #6808 into master will decrease coverage by 1%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6808      +/-   ##
============================================
- Coverage      55.6%   54.59%   -1.01%     
+ Complexity    10547     9566     -981     
============================================
  Files           774      712      -62     
  Lines         88523    83213    -5310     
  Branches       8080     7482     -598     
============================================
- Hits          49221    45431    -3790     
+ Misses        37083    35748    -1335     
+ Partials       2219     2034     -185

Impacted Files	Coverage Δ	Complexity Δ
.../cloud/datastore/testing/LocalDatastoreHelper.java	80.59% <0%> (-4.48%)	17% <0%> (ø)
.../com/google/cloud/pubsub/v1/MessageDispatcher.java	84.61% <0%> (-0.52%)	23% <0%> (ø)
...le/cloud/firestore/v1/stub/FirestoreAdminStub.java
...m/google/cloud/firestore/v1/FirestoreSettings.java
...loud/firestore/v1beta1/stub/GrpcFirestoreStub.java
...cloud/firestore/v1/stub/FirestoreStubSettings.java
.../firestore/v1/stub/FirestoreAdminStubSettings.java
.../firestore/v1beta1/stub/FirestoreStubSettings.java
...e/cloud/firestore/collection/LLRBRedValueNode.java
...oogle/cloud/firestore/spi/v1/GrpcFirestoreRpc.java
... and 22 more

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[BenWhitehead]

Thanks @schmidt-sebastian! I've rebased and pushed.

The code over in java-firestore is up-to-date as of the MR I merged a little while ago.