examples/flask_sqlalchemy: dependency specification needed, due to breaking change in graphql-core ^3

The requirements should be updated so that Flask-GraphQL==2.3.0 is used, elsewhise pip will install Flask-GraphQL>=3 and this will lead to an import error in graphql-server, because get_default_backend is not defined in graphql/graphql-core>=3 . This of course won't be necessary if the deeper lying dependency issues are fixed, but while they are present, this is a quick fix.

Start in the examples/flask_sqlalchemy example and locate its dependency requirements. Pin Flask-GraphQL to 2.3.0, then verify that the example's dependency specification prevents the reported graphql-server import error; no specific test is mentioned in the issue.

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