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".

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

Derived from GoogleCloudPlatform/magic-modules#4702