Use indexmap version 1.6.2 in async-graphql-value

[terahlunah]

Issue

indexmap 1.7.0 is causing dependency cycles when used alongside certains crates (see tkaitchuck/aHash#95)
Currently, the only fix is to downgrade to indexmap 1.6.2

async-graphql depends on indexmap = "1.6.2"
but async-graphql-value depends on indexmap = "1.7.0"

This is preventing me pinning indexmap to 1.6.2 in my project

Proposed solution

Change async-graphql-value to depend on indexmap = "1.6.2", like async-graphql

No other change is required

[sunli829]

Thanks!

[sunli829]

Thanks!