Made billing budget amount updatable

[melinath]

Billing budget amount was not actually getting updated. Resolved hashicorp/terraform-provider-google#9440

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)

billing: made `amount.specified_amount.units` updatable on `google_billing_budget`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 48 insertions(+), 5 deletions(-))
Terraform Beta: Diff ( 2 files changed, 49 insertions(+), 6 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 51 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 2 files changed, 51 insertions(+), 6 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccBillingBudget_billingBudgetUpdate|TestAccContainerNodePool_withGPU You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=194281

[modular-magician]

Tests failed during RECORDING mode: TestAccContainerNodePool_withGPU Please fix these to complete your PR

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccBillingBudget_billingBudgetUpdate|TestAccContainerNodePool_withGPU You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=194284

[modular-magician]

Tests failed during RECORDING mode: TestAccContainerNodePool_withGPU Please fix these to complete your PR

[melinath]

Test failure is unrelated.