Use 'google.golang.org/protobuf' instead of 'github.com/golang/protobuf/' as much as possible

[mmorel-35]

RELEASE NOTES: none

Signed-off-by: Matthieu MOREL matthieu.morel35@gmail.com

[codecov]

Codecov Report

Merging #7004 (a2240db) into master (5ccf176) will decrease coverage by 0.15%.
The diff coverage is 87.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7004      +/-   ##
==========================================
- Coverage   82.43%   82.29%   -0.15%     
==========================================
  Files         296      296              
  Lines       31473    31455      -18     
==========================================
- Hits        25946    25887      -59     
- Misses       4468     4500      +32     
- Partials     1059     1068       +9     

Files	Coverage Δ
channelz/service/func_linux.go	85.36% <100.00%> (ø)
channelz/service/service.go	83.54% <100.00%> (-1.17%)	⬇️
xds/internal/xdsclient/bootstrap/bootstrap.go	74.48% <33.33%> (ø)

... and 17 files with indirect coverage changes

[dfawley]

The changes to channelz will conflict with #6969, so I'd rather do them later (as that is a huge PR), and the other changes you've made are already included in #6961.

Thank you for the PR, though!