Fix bug where vault_kv_secret_v2 resource's cas field had no effect

[busser]

This PR fixes a bug described in issue #1727. The vault_kv_secret_v2 resource's cas field's value was misplaced in the payload sent to Vault, failing to create or update secrets in an engine that required the check-and-set parameter.

This PR also updates the resource's acceptance test by using the cas field. Running the test without the fix reproduces the error described in the issue.

Community Note

• Please vote on this pull request by adding a 👍 reaction to the original pull request comment to help the community and maintainers prioritize this request
• Please do not leave "+1" comments, they generate extra noise for pull request followers and do not help prioritize the request

Closes #1727

Release note for CHANGELOG:

Fix a bug where the `vault_kv_secret_v2` resource's `cas` field had no effect.

Output from acceptance testing:

export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="******"
$ make testacc TESTARGS='-run=TestAccKVSecretV2'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAccKVSecretV2 -timeout 30m ./...
?   	github.com/hashicorp/terraform-provider-vault	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/generate	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/codegen	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/generated	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/helper	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/consts	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/internal/identity/entity	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/internal/identity/group	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/identity/mfa	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/pki	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/internal/provider	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/schema	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/testutil	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/util	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/vault	2.198s

[busser]

Closing this PR. It's been open for close to two years and hasn't received a response.