Updates User API attribute is-site-admin to leverage is-admin

[mfhodges]

Description

The User API will be deprecating the is-site-admin attribute in favor of is-admin which was introduced yesterday

This is a one-line change since this variable is being referenced anywhere yet in go-tfe.

Docs updates https://github.com/hashicorp/terraform-docs-common/pull/636 will be merged once the next release is cut

Testing plan

External links

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ TFE_ADDRESS="https://example" TFE_TOKEN="example" go test ./... -v -run TestFunctionsAffectedByChange

...

[brandonc]

Let's add a CHANGELOG entry as well. Be sure to include it under a header called ## Deprecations

[github-actions[bot]]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.