feat(generator): update protoc to v3.15.3

[yoshi-automation]

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PiperOrigin-RevId: 359781040
Source-Link: googleapis/googleapis@f6dd7e4

[codecov]

Codecov Report

Merging #551 (c6e6a9a) into master (ed6b798) will decrease coverage by 0.64%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #551      +/-   ##
============================================
- Coverage     78.85%   78.20%   -0.65%     
  Complexity      376      376              
============================================
  Files            27       27              
  Lines          3414     3414              
  Branches        171      169       -2     
============================================
- Hits           2692     2670      -22     
- Misses          650      652       +2     
- Partials         72       92      +20     

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsub/v1/SubscriptionAdminClient.java	57.65% <0.00%> (-5.41%)	52.00% <0.00%> (ø%)
...om/google/cloud/pubsub/v1/SchemaServiceClient.java	57.47% <0.00%> (-3.45%)	20.00% <0.00%> (ø%)
...gle/cloud/pubsub/v1/SequentialExecutorService.java	87.77% <0.00%> (-3.34%)	0.00% <0.00%> (ø%)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	56.64% <0.00%> (-2.32%)	34.00% <0.00%> (ø%)

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