fix(pubsub): increase backoff from 1.3 to 4.0

[alevenberg]

Fixes #12328

This change is 

[dbolduc]

This is not a feat. Maybe it is a fix? Although I have no idea what we are fixing...

Maybe put the motivation for this change in the PR description?

[alevenberg]

Discussed with Darren.

Closing since the motivating change is due to how the GAPIC libraries implement exponential backoff.

C++ does not use that implementation.

[alevenberg]

Re-opening, the pub/sub team will create an issue motivating the change

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2fc5960) 93.59% compared to head (172501e) 93.59%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12306   +/-   ##
=======================================
  Coverage   93.59%   93.59%           
=======================================
  Files        2026     2026           
  Lines      179055   179055           
=======================================
+ Hits       167578   167581    +3     
+ Misses      11477    11474    -3     

Files Changed	Coverage Δ
google/cloud/pubsub/internal/defaults.cc	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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