Ignored case changes in bigquery table schema mode/type

[modular-magician]

BigQuery table schema normalizes the mode and type to uppercase, which causes a permadiff for the end user if they set it in lowercase.

Also removed tests for setting type to nil, because a field type being nil is invalid.

Resolved #9472

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Release Note Template for Downstream PRs (will be copied)

bigquery: Fixed permadiff due to lowercase mode/type in `google_bigquery_table.schema`

Derived from GoogleCloudPlatform/magic-modules#4943

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