spanner-jdbc: Add beta note to README and cleanup pom.xml

[kolea2]

No description provided.

[kolea2]

@chingor13 please take another quick look. Thanks!

[codecov]

Codecov Report

Merging #6047 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6047      +/-   ##
============================================
+ Coverage     47.37%   47.38%   +0.01%     
- Complexity    27111    27118       +7     
============================================
  Files          2516     2516              
  Lines        273718   273815      +97     
  Branches      31353    31354       +1     
============================================
+ Hits         129670   129754      +84     
- Misses       134440   134452      +12     
- Partials       9608     9609       +1

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/vision/v1/ProductSearchSettings.java	10.14% <0%> (-0.63%)	2% <0%> (ø)
...google/cloud/vision/v1/stub/ProductSearchStub.java	3.7% <0%> (-0.3%)	1% <0%> (ø)
...le/cloud/vision/v1/stub/GrpcProductSearchStub.java	96.86% <0%> (-0.06%)	27% <0%> (+1%)
...om/google/cloud/vision/v1/ProductSearchClient.java	61.34% <0%> (-0.05%)	80% <0%> (+4%)
...loud/vision/v1/stub/ProductSearchStubSettings.java	83.15% <0%> (+1.22%)	31% <0%> (+2%)	⬆️
...e/cloud/vision/v1/stub/GrpcImageAnnotatorStub.java	90.51% <0%> (+1.97%)	12% <0%> (ø)	⬇️
...gle/cloud/storage/testing/RemoteStorageHelper.java	65.04% <0%> (ø)	9% <0%> (ø)	⬇️
.../cloud/datastore/testing/LocalDatastoreHelper.java	85.07% <0%> (ø)	17% <0%> (ø)	⬇️
.../java/com/google/cloud/testing/CommandWrapper.java	96.96% <0%> (ø)	13% <0%> (ø)	⬇️

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