Validation/coercion of input variables violates the GraphQL spec

When callling graphql-core's `execute()` function with `variable_values` which do not pass validation -- for example, including an unexpected key in the `variable_values` dictionary -- the current behavior is that an `ExecutionResult` object is returned from the function, with the associated GraphQLError present inside it. Instead, this should be treated as a Request error, according to the spec, meaning that a `GraphQLError` should be raised from `execute()`.

The GraphQL spec states:

> [Request errors](https://spec.graphql.org/October2021/#sec-Errors.Request-errors)
Request errors are raised before execution begins. This may occur due to a parse grammar or validation error in the requested document, an inability to determine which operation to execute, or **invalid input values for variables**.

This means that it is incorrect for the `coerce_variable_values` function to be returning a GraphQLError inside an ExecutionResult: https://github.com/graphql-python/graphql-core/blob/9dcf25e66f6ed36b77de788621cf50bab600d1d3/src/graphql/execution/values.py#L93-L99

doing so means that a response payload is returned containing both an "errors" key and a null "data" key. Again, this a violation of the spec:

> If a request error is raised, execution does not begin and the **data entry in the response must not be present**. The errors entry must include the error.