provider: Add sts_region

[YakDriver]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TESTARGS="-run TestAccProvider" PKG_NAME="internal/acctest"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/acctest/... -v -count 1 -parallel 20  -run TestAccProvider -timeout 180m
--- PASS: TestAccProvider_Region_awsSC2S (28.88s)
--- PASS: TestAccProvider_Region_stsRegion (29.69s)
--- PASS: TestAccProvider_Region_awsCommercial (31.08s)
--- PASS: TestAccProvider_Region_awsChina (31.22s)
--- PASS: TestAccProvider_Region_awsC2S (31.98s)
--- PASS: TestAccProvider_Region_awsGovCloudUs (31.99s)
--- PASS: TestAccProvider_DefaultTagsTags_one (38.09s)
--- PASS: TestAccProvider_IgnoreTagsKeyPrefixes_one (38.54s)
--- PASS: TestAccProvider_IgnoreTagsKeys_none (38.55s)
--- PASS: TestAccProvider_IgnoreTagsKeyPrefixes_none (38.56s)
--- PASS: TestAccProvider_DefaultAndIgnoreTags_emptyBlocks (39.05s)
--- PASS: TestAccProvider_IgnoreTagsKeys_one (39.49s)
--- PASS: TestAccProvider_IgnoreTagsKeyPrefixes_multiple (39.49s)
--- PASS: TestAccProvider_DefaultTagsTags_multiple (39.75s)
--- PASS: TestAccProvider_DefaultTags_emptyBlock (39.85s)
--- PASS: TestAccProvider_IgnoreTags_emptyBlock (39.94s)
--- PASS: TestAccProvider_IgnoreTagsKeys_multiple (40.22s)
--- PASS: TestAccProvider_endpoints (41.17s)
--- PASS: TestAccProvider_AssumeRole_empty (43.23s)
--- PASS: TestAccProvider_DefaultTagsTags_none (18.03s)
--- PASS: TestAccProvider_unusualEndpoints (18.84s)
--- PASS: TestAccProvider_fipsEndpoint (49.48s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/acctest	51.785s

[gdavison]

Looks good. sts_region should also be documented in the v4 upgrade guide

[github-actions]

This functionality has been released in v4.2.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.