deps: update dependency com.google.protobuf:protobuf-bom to v3.15.7

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.protobuf:protobuf-bom (source)	3.15.6 -> 3.15.7

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Release Notes

protocolbuffers/protobuf

v3.15.7

Compare Source

C++

• Remove the ::pb namespace (alias) (#​8423)

Ruby

• Fix unbounded memory growth for Ruby <2.7 (#​8429)
• Fixed message equality in cases where the message type is different (#​8434)

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[codecov]

Codecov Report

Merging #413 (bf67969) into master (b573351) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #413   +/-   ##
=========================================
  Coverage     67.74%   67.74%           
  Complexity      384      384           
=========================================
  Files            36       36           
  Lines          1981     1981           
  Branches        267      267           
=========================================
  Hits           1342     1342           
  Misses          531      531           
  Partials        108      108           

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