Remove validation on Cloud Functions available_memory_mb

[modular-magician]

I saw a question about whether we support 8GB (8192) of memory or not because it was introduced in the API, and we don't- these values have changed out of band a few times, and I don't feel that this validation is helpful enough to be worth keeping up with value churn. Also note that our docs never reflected supporting 4GB- I figure it's clear enough what the pattern for 2048/4096/8192 will be, so I just changed values past 1024 to "etc".

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)

cloudfunctions: removed bounds on the supported memory range in `google_cloudfunctions_function.available_memory_mb`

Derived from GoogleCloudPlatform/magic-modules#4702

[ghost]

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