release: update dgo module from v200 to v210 (dgraph-io#148)

Upgrades module path from v200 to v210.
Note that there is nothing BREAKING in this version. All the changes are backward compatible. We have added a few new APIs since v200.03.0 release.

DialSlashEndpoint and few more related to auth/slash (these are deprecated and will be removed in later releases)
LoginIntoNamespace
Relogin
GetJwt
This PR additionally:

Updates the README with Login example.
Add deprecation comment for DialSlash* APIs

CHANGELOG

=========

- Use background flag for computing indexes in background (dgraph-io#118)

Fix ACL tests to work with Dgraph 1.2.0

Simplify type system test cases (dgraph-io#101)

Ref: hypermodeinc/dgraph#4017

Minor change in docs (dgraph-io#89)

breaking change: Merge Mutate and Query endpoint into Query endpoint. (…

…dgraph-io#77)

* Add Upsert function
* One endpoint for running queries and mutations
* Remove deprecated fields from various structs

Update README (dgraph-io#80)