Merge branch 'rc/1.9.20' into release

Merge branch 'rc/1.9.19' into release

Merge branch 'rc/1.9.18' into release

Merge branch 'rc/1.9.17' into release

Merge branch 'rc/1.9.16' into release

Merge branch 'rc/1.9.15' into release

Downgrade Linux GitHub Actions environment to fix error (google#3588)

Merge branch 'rc/1.9.13' into release

Merge branch 'rc/1.9.12' into release

Merge branch 'rc/1.9.11' into release