Validate the size of the value for instance level variables

What does this MR do?

related issue #217936 (closed)

Instance level variables have a limit of 1024 for the encrypted value at the database level. The corresponding validation on the model was missing resulting in 500 errors when trying to create variables with values above it.

Because the encrypted_value is base64 encoded, we don't get to use the whole 1024 chars for the value.

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Does this MR meet the acceptance criteria?

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