Update github.com/golang/protobuf and google.golang.org/protobuf modules

[shashkin]

Updated to latest version to catch up descriptor changes
Also regenerated files after grpc/grpc-proto#146

RELEASE NOTES: N/A

[codecov]

Codecov Report

Merging #7028 (df13010) into master (f7c5e6a) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7028      +/-   ##
==========================================
- Coverage   82.48%   82.46%   -0.02%     
==========================================
  Files         296      296              
  Lines       31468    31468              
==========================================
- Hits        25955    25951       -4     
- Misses       4457     4460       +3     
- Partials     1056     1057       +1     

see 15 files with indirect coverage changes

[arvindbr8]

Thanks for the PR @shashkin -- We need to wait for a second approval from a maintainer, per policy.