Remove custom OIDC limitation for Play with Self-Managed in 8.9

[giorgionaps]

In the 8.9 docs, remove the limitation stating that “Play does not work with a custom OIDC provider” for Self-Managed and update the Use Play with Camunda Self-Managed section (and any other occurrences) to reflect that custom OIDC is now supported:

https://docs.camunda.io/docs/next/components/modeler/web-modeler/validation/play-your-process/#use-play-with-camunda-self-managed