Merge pull request grpc#11283 from jtattermusch/bump_136

bump version to 1.3.6

1.3.5 release1.3.5 release1.3.5 release1.3.5 release1.3.5 release

1.3.3 release1.3.3 release1.3.3 release

Merge pull request grpc#11250 from matt-kwong/undo_proto_change

Undo downgrade of Protobuf submodule

Merge pull request grpc#11073 from stanley-cheung/bump-to-1_3_2

Bump to version 1.3.2

Merge pull request grpc#10990 from murgatroid99/version_1.3.1_bump

Update version to 1.3.1

s/1.3.0-pre/1.3.0

Merge pull request grpc#10738 from apolcyn/bump_12x_to_125

bumpy v1.2.x branch to v 1.2.5

Merge pull request grpc#10606 from murgatroid99/version_bump_1.2.4

Bump version to 1.2.4

Merge pull request grpc#10520 from murgatroid99/bump_1.2.3

Bump version to 1.2.3