Merge pull request grpc#15895 from srini100/v1.13.x

Bump to v1.13.0

Merge pull request grpc#15849 from mehrdada/bump-rc3

Bump rc3

Merge pull request grpc#15783 from mehrdada/bump-v1.13.x-to-pre2

Bump v1.13.x to pre2

Merge pull request grpc#15697 from srini100/backport15690

Backport grpc#15690 from master to v1.13.x

Fixes:

grpc#15071
grpc#15545

Merge pull request grpc#15374 from mehrdada/v1.12.x

Bump v1.12.x to v1.12.0

Merge pull request grpc#15371 from mehrdada/v1.11.x

Backport 15362 to 1.11.x

Merge pull request grpc#15263 from muxi/backport-15262

Backport 15262: Provide protocol for initializer of generated messages

Merge pull request grpc#15257 from mehrdada/v1.11.x

Bump v1.11.x to v1.11.1-pre1

Merge pull request grpc#15035 from mehrdada/v1.11.x

Bump v1.11.x to 1.11.0   #gorgeous