[Bug Fix] - azuredevops_variable_group - Exclude disabled secrets

[xuzhang3]

All Submissions:

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

What about the current behavior has changed?

1. Exclude disabled secrets.
2. Fix LastRefreshedOn null issue.

Issue Number: #945

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

• Yes
• No

Does this introduce a breaking change?

• Yes
• No

Any relevant logs, error output, etc?

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

Other information

[xuzhang3]

=== RUN   TestAccVariableGroup_DataSource
=== PAUSE TestAccVariableGroup_DataSource
=== RUN   TestAccVariableGroup_DataSourceKeyVault
=== PAUSE TestAccVariableGroup_DataSourceKeyVault
=== RUN   TestAccVariableGroup_CreateAndUpdate
=== PAUSE TestAccVariableGroup_CreateAndUpdate
=== RUN   TestAccVariableGroupKeyVault_CreateAndUpdate
=== PAUSE TestAccVariableGroupKeyVault_CreateAndUpdate
=== CONT  TestAccVariableGroup_DataSource
=== CONT  TestAccVariableGroup_CreateAndUpdate
=== CONT  TestAccVariableGroupKeyVault_CreateAndUpdate
=== CONT  TestAccVariableGroup_DataSourceKeyVault
--- PASS: TestAccVariableGroup_DataSource (62.26s)
--- PASS: TestAccVariableGroup_DataSourceKeyVault (83.80s)
--- PASS: TestAccVariableGroupKeyVault_CreateAndUpdate (85.04s)
--- PASS: TestAccVariableGroup_CreateAndUpdate (93.84s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        101.708s

[xuzhang3]

=== RUN   TestAccVariableGroup_DataSource
=== PAUSE TestAccVariableGroup_DataSource
=== RUN   TestAccVariableGroup_DataSourceKeyVault
=== PAUSE TestAccVariableGroup_DataSourceKeyVault
=== RUN   TestAccVariableGroup_CreateAndUpdate
=== PAUSE TestAccVariableGroup_CreateAndUpdate
=== RUN   TestAccVariableGroupKeyVault_CreateAndUpdate
=== PAUSE TestAccVariableGroupKeyVault_CreateAndUpdate
=== CONT  TestAccVariableGroup_DataSource
=== CONT  TestAccVariableGroup_CreateAndUpdate
=== CONT  TestAccVariableGroupKeyVault_CreateAndUpdate
=== CONT  TestAccVariableGroup_DataSourceKeyVault
--- PASS: TestAccVariableGroup_DataSource (83.84s)
--- PASS: TestAccVariableGroup_DataSourceKeyVault (109.27s)
--- PASS: TestAccVariableGroupKeyVault_CreateAndUpdate (112.99s)
--- PASS: TestAccVariableGroup_CreateAndUpdate (144.74s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        153.453s