Upgrade module to v21

Upgrade module to v20

Update version to v19 (google#1056)

Support updates to Marketplace APIs and webhook payload (google#1042)

Helps google#1040.

Restrict request params of CreateHook for repos and orgs to known sub…

…set (google#1018)

Fixes google#1015.

Restrict request params of CreateRelease and EditRelease to known sub…

…set (google#997)

Fixes google#992.

Introduce Go modules and remove support for Go 1.8 (google#985)

Update orgs_members.go (google#871)

Installation: Use *Timestamp instead of *time.Time (google#965)

Migrate IDs to int64 type (google#816)

A helper function "Int64" is also implemented to assist the migration of
unit tests.

Refer google#597.