Remove typing aliases deprecated in python 3.9. Also replace usages of typing.Union/Optional with | (#5277)

* Remove typing aliases deprecated in python 3.9

* Remove usages of Union

* Update codegen/opentelemetry-codegen-json/src/opentelemetry/codegen/json/generator.py

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>

* Replace all usages of typing.Optional with |

* Create 5277.changed

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Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>