artifactregistry: remove repository.mode plan-time validation

[Subserial]

Changes mode field type from enum to string, removing enum value validation.

Fixes hashicorp/terraform-provider-google#23234

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artifactregistry: remove plan-time validation for `mode` on `google_artifact_registry_repository`

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 1b46160:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	2 files changed, 5 insertions(+), 8 deletions(-)
google-beta provider	View Diff	2 files changed, 5 insertions(+), 8 deletions(-)
terraform-google-conversion	View Diff	1 file changed, 5 insertions(+), 6 deletions(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
55	45	10	0

Affected Service Packages

• artifactregistry

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@Subserial, @MeBaranov, @SirGitsalot VCR tests complete for 1b46160!