Remove dependency on `github.com/hashicorp/vault` package

[vinay-gopalan]

Description

Now that https://github.com/hashicorp/vault/pull/25744 has been merged, we are able to complete the updates needed to remove the dependency on the vault package. This PR removes the package dependency and has additional/related changes:

• adds in constants and test helpers needed to the consts and testutil packages
• upgrades the Vault API and SDK packages to their latest versions

[fairclothjm]

Looks like removing the Vault package saves us about 4M too!

$ git checkout main && make dev > /dev/null && du -sh ~/.terraform.d/plugins/terraform-provider-vault
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
 50M    /Users/jmf/.terraform.d/plugins/terraform-provider-vault

$ git checkout VAULT-26362/remove-vault-pkg-dep && make dev > /dev/null && du -sh ~/.terraform.d/plugins/terraform-provider-vault
Switched to branch 'VAULT-26362/remove-vault-pkg-dep'
Your branch is up to date with 'origin/VAULT-26362/remove-vault-pkg-dep'.
 46M    /Users/jmf/.terraform.d/plugins/terraform-provider-vault