Remove "locked" flag of DNS records

[janik-cloudflare]

This flag hasn't been used in quite a while and we're preparing to remove it from the API.

[github-actions]

changelog detected ✅

[jacobbednarz]

acceptance tests all passing

TF_ACC=1 go test ./internal/sdkv2provider -v -run "^TestAccCloudflareRecordDataSource" -count 1 -timeout 120m -parallel 1
=== RUN   TestAccCloudflareRecordDataSource
--- PASS: TestAccCloudflareRecordDataSource (10.39s)
=== RUN   TestAccCloudflareRecordDataSourceTXT
--- PASS: TestAccCloudflareRecordDataSourceTXT (7.37s)
=== RUN   TestAccCloudflareRecordDataSourceMX
--- PASS: TestAccCloudflareRecordDataSourceMX (8.60s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/sdkv2provider	27.324s

[github-actions]

This functionality has been released in v4.29.0 of the Terraform Cloudflare 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!