Bug fix `azuredevops_team` - Add members should not fail if the member is already present

[nilleb]

When adding a member to a group, If this member is already part of the group, We should be considering the mode specified on the resource And NOT fail if the user is already member of the group.

All Submissions:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] My code follows the code style of this project.
• [x] I ran lint checks locally prior to submission.
• [x] Have you checked to ensure there aren't other open PRs for the same update/change?

What about the current behavior has changed?

Issue Number: #1129

Does this introduce a change to go.mod, go.sum or vendor/?

• [ ] Yes
• [x] No

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Any relevant logs, error output, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information

[nilleb]

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[nilleb]

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[xuzhang3]

=== RUN   TestAccTeam_DataSource_Basic
--- PASS: TestAccTeam_DataSource_Basic (88.53s)
=== RUN   TestAccTeam_CreateAndUpdate
--- PASS: TestAccTeam_CreateAndUpdate (108.87s)
=== RUN   TestAccTeam_CreateAndUpdateAdministrators
--- PASS: TestAccTeam_CreateAndUpdateAdministrators (153.27s)
=== RUN   TestAccTeam_CreateAndUpdateMembers
--- PASS: TestAccTeam_CreateAndUpdateMembers (160.57s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        517.293s

[xuzhang3]

@nilleb LGTM