fix: mark v1beta1 as deprecated

[schmidt-sebastian]

We would like to discourage use of the v1beta1 protocol.

Port of googleapis/nodejs-firestore#937

[schmidt-sebastian]

cc @BenWhitehead

[codecov]

Codecov Report

Merging #154 into master will increase coverage by 0.63%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #154      +/-   ##
============================================
+ Coverage     71.62%   72.25%   +0.63%     
- Complexity      969      978       +9     
============================================
  Files            62       62              
  Lines          5222     5222              
  Branches        579      579              
============================================
+ Hits           3740     3773      +33     
+ Misses         1302     1268      -34     
- Partials        180      181       +1

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/firestore/v1beta1/FirestoreSettings.java	13.2% <ø> (ø)	2 <0> (ø)	⬇️
.../firestore/v1beta1/stub/FirestoreStubSettings.java	82.02% <ø> (+2.76%)	22 <0> (ø)	⬇️
...le/cloud/firestore/v1beta1/stub/FirestoreStub.java	6.25% <ø> (ø)	1 <0> (ø)	⬇️
...loud/firestore/v1beta1/stub/GrpcFirestoreStub.java	96.04% <ø> (+2.37%)	19 <0> (+3)	⬆️
...oogle/cloud/firestore/v1beta1/FirestoreClient.java	76.28% <ø> (+21.64%)	31 <0> (+6)	⬆️
...ore/v1beta1/stub/GrpcFirestoreCallableFactory.java	62.5% <ø> (ø)	5 <0> (ø)	⬇️

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