mark redis instance maintenanceSchedule block as output only

[modular-magician]

For some reason maintenance_schedule is being set (to NULL) even though the whole block is output-only.

I haven't been able to figure out with 100% certainty why, but me and others, are suddenly suffering from this.

I think it might have to do with a slight change in the gcloud API regarding scheduled upcoming maintenance...

for instances that are due for an upgrade the gcloud API will return the maintenance_schedule for the scheduled upcoming maintenance, and then terraform incorrectly NULLs the 3 properties because they're output only, leaving an empty maintenance_schedule object that it will use apply.

for a freshly creation redis instance I don't get a maintenanceSchedule in the GET GET /v1beta1/projects/my-instance/locations/europe-west4/instances/aurora-redis?alt=json while for an older one it is present.

note how it nulls the properties, not the whole object;

      - maintenance_schedule {
          - end_time               = "2023-07-08T05:00:00Z" -> null
          - schedule_deadline_time = "2023-07-15T04:00:00Z" -> null
          - start_time             = "2023-07-08T04:00:00Z" -> null
        }

This PR marks the whole block as output: true to by-pass this problem entirely.

Fixes 11871

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 Write release notes before writing my release note below.

Note: I wasn't able to run the acceptance test because I'm struggling with the authentication ... will try to resolve ...

Release Note Template for Downstream PRs (will be copied)

fixed issue with `google_redis_instance` populating output-only field `maintenance_schedule`. 

Derived from GoogleCloudPlatform/magic-modules#8261

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