Merge pull request grpc#23577 from jtattermusch/bump_1_31_0_pre1

bump version to v1.31.0-pre1 (on v1.31.x branch)

Merge pull request grpc#23520 from stanley-cheung/bump-version-1-30-2

Bump version to 1.30.2

Merge pull request grpc#23405 from apolcyn/bump_version

Bump 1.30.x branch to 1.30.1

Merge pull request grpc#23266 from srini100/v1.30.x

Bump to v1.30.0

Merge pull request grpc#23155 from srini100/v1.30.x

bump v1.30.x branch  to v1.30.0-pre1

Merge pull request grpc#23014 from stanley-cheung/bump-1-29-for-php

Bump version to 1.29.1 for PHP

Merge pull request grpc#23010 from stanley-cheung/bump-1-28-for-objc

Bump version to 1.28.2 for Obj-C

Flipping version.

Merge pull request grpc#22563 from ericgribkoff/cherry_pick_pip

Merge pull request grpc#22560 from ericgribkoff/update_pip

Merge pull request grpc#22537 from gnossen/cherrypick_22525

Backport grpc#22525 to 1.28 (Report RPC failures to LB recv_trailing_metadata callbacks.)