Replacing explicit read access for external contributors with team membership

We're moving away from using individual permissions to our repos in favor of teams. In the past, we've given external contributors explicit read access, for two reasons:

1. To assign them issues they would like to work on
2. To give them access to CI logs in Azure DevOps pipelines

The first one is no longer necessary as GitHub has made a change that allows anyone to be assigned an issue so long they either have explicit permissions or have commented on the issue. The second one is still important so that contributors who submitted PRs can access the detailed logs in order to figure out why their PR didn't pass CI.

In order to make our lives easier, we decided to move the individual read permissions to team memberships. This requires the users to accept the invitation to the dotnet org. To ensure the impacted users are aware of this, we have tagged them in the [corresponding discussion][disc]. Please note that we only grant read access on a case-by-case basis, usually for non-trivial PRs.

### Discussion

For a discussion, please see the corresponding [discussion issue][disc].

[disc]: https://github.com/dotnet/core/issues/6722

No repository files, tests, or implementation entry points are identified. Start by reading the corresponding discussion issue at github.com/dotnet/core/issues/6722 and the announcement's description of Azure DevOps pipeline access; done means the permission change and contributor invitations are handled through team memberships.

Written by the indexing model from the issue text.