Allow ENTERPRISE_PLUS as default Edition for POSTGRES_16

[modular-magician]

Current default: ENTERPRISE for settings.edition is not consistent with the API which treats EDITION_UNSPECIFIED as the default (refer link) when customer doesn't provide any. With PostgreSQL version 16, the default Edition will be ENTERPRISE_PLUS as documented in the PostgreSQL June 07, 2024 release notes. Thus, unset the default edition so that it can instead be computed in the API, allowing the API to use ENTERPRISE_PLUS as the default for POSTGRES_16.

b/338258663

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• 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).
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Release Note Template for Downstream PRs (will be copied)

sql: removed the client-side default of `ENTERPRISE` for `edition` in `google_sql_database_instance` so that `edition` is determined by the API when unset. This will cause new instances to use `ENTERPRISE_PLUS` as the default for POSTGRES_16.

Derived from GoogleCloudPlatform/magic-modules#12077

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