deps: update dependency com.google.guava:guava-bom to v28.2-android

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
com.google.guava:guava-bom	minor	28.1-android -> 28.2-android

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[suztomo]

Memo for later reference:

Kokoro - Test: Dependencies — Build failed:

[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce) @ grpc-google-cloud-websecurityscanner-v1alpha ---
[WARNING] Rule 2: org.apache.maven.plugins.enforcer.RequireUpperBoundDeps failed with message:
Failed while enforcing RequireUpperBoundDeps. The error(s) are [
Require upper bound dependencies error for com.google.errorprone:error_prone_annotations:2.3.3 paths to dependency are:
+-com.google.api.grpc:grpc-google-cloud-websecurityscanner-v1alpha:0.81.1-SNAPSHOT
  +-io.grpc:grpc-api:1.26.0
    +-com.google.errorprone:error_prone_annotations:2.3.3
and
+-com.google.api.grpc:grpc-google-cloud-websecurityscanner-v1alpha:0.81.1-SNAPSHOT
  +-com.google.guava:guava:28.2-android
    +-com.google.errorprone:error_prone_annotations:2.3.4
]

[codecov]

Codecov Report

Merging #32 into master will decrease coverage by 0.71%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #32      +/-   ##
============================================
- Coverage      76.3%   75.59%   -0.72%     
  Complexity      208      208              
============================================
  Files            12       12              
  Lines          1680     1680              
  Branches         22       10      -12     
============================================
- Hits           1282     1270      -12     
  Misses          352      352              
- Partials         46       58      +12

Impacted Files	Coverage Δ	Complexity Δ
...urityscanner/v1alpha/WebSecurityScannerClient.java	63.59% <0%> (-2.92%)	55% <0%> (ø)
...curityscanner/v1beta/WebSecurityScannerClient.java	63.59% <0%> (-2.92%)	55% <0%> (ø)

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[chingor13]

This combined with #48 passes the dependency check