Fix sql_database_instance creation to remove root user earlier

[modular-magician]

b/254877908

This fixes a bug where a root user without a password would sometimes remain on an sql_database_instance after creation, creating a security risk. The logic in the provider is meant to remove this root user at the end of the creation function, but if there is a failure before then, the instance is still created but the root user remains.

The fix here simply performs the root user removal earlier, so that it is done as soon as it is not longer needed. This eliminates a subset of failure modes that would have left the root user on the instance. I believe this is the best we can do for now, short of more elaborate retry logic.

For testing, it was very difficult to come up with a test that would verify this problem has been resolved, because triggering arbitrary errors is nontrivial, and our test harness does not offer a clean way to check the state of the environment after an error occurs. I've come up with work-arounds to produce a test that I believe is adequate here.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

sql: fixed `sql_database_instance` leaking root users

Derived from GoogleCloudPlatform/magic-modules#6922

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