Remove collation from update request and make collation non-updatable

[modular-magician]

Fixes #10959
Fixes #11947

update API does not accept this field at all during updates, even if it is not changing.
b/216858127

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

sql: fixed an issue in `google_sql_database_instance` where updates would fail because of the `collation` field

Derived from GoogleCloudPlatform/magic-modules#6298

[github-actions]

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