Deleted immutable attributes for initialUser and weeklySchedule so th…

[modular-magician]

…at deleting/adding these properties doesn't force new cluster creation

Creating a cluster with the minimum fields works fine the first time. On re-running the same .tf file, it deletes the existing cluster and creates a new one.

We get the default values for automated backup policy from the API. This includes many fields. One of them is the weekly schedule. This field is marked as forceNew. The first time when we create the cluster without providing any automated backup policy, we get the default from the API. This default value gets persisted in the .tfstate file. On subsequent runs of terraform, Terraform sees that the automated backup policy is present in the state file but not in the .tf file. Since this field is marked as forceNew, the old cluster is deleted and a new one is created. This is highly undesirable.

Similarly, initialUser is marked as immutable while it is a mutable field. Addition/Deletion to this field causes existing cluster deletion and creation of a new one which is highly undesirable.

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: Deleted immutable flag for initalUser and weeklySchedule 

Derived from GoogleCloudPlatform/magic-modules#7552

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