Make cluster location a required field

[modular-magician]

Cluster location is marked as optional. It should be marked as required because we might extend the regional API to zonal and therefore can't use the provider default region/environment variable for region.

Solutions considered:

1. Inherit the provider location/region
   • It doesn't work right now because it is not added in ProviderConfig
   • It is undesirable as it restricts us from extending the API from regional to zonal.
2. Get the location from environment variables
   • Same issue as non-extensibility.
3. Mark Location as a required field
   • Ensures extensibility of the APIs in future.

Solution accepted: Mark location as a required field.

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (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)

alloydb: changed `location` from `optional` to `required` for `google_alloydb_cluster`. `location` had previously been marked as optional, but operations failed if it was omitted, and there was no way for `location` to be inherited from the provider configuration or from an environment variable. This means there was no way to have a working configuration without `location` specified.

alloydb: changed `location` from `optional` to `required` for `google_alloydb_cluster`. `location` had previously been marked as optional, but operations failed if it was omitted, and there was no way for `location` to be inherited from the provider configuration or from an environment variable. This means there was no way to have a working configuration without `location` specified.

Derived from GoogleCloudPlatform/magic-modules#7652

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.