Update docs to mention that username is not supported for USER authorizations on SaaS

[YuliaKorsun]

Based on the slack thread: link

Orchestration Cluster authorization mentions that USER authorization accepts *, felix.mueller--> All users / Username for Resource key type https://docs.camunda.io/docs/next/components/concepts/access-control/authorizations/#available-resources

While in SaaS only emails can grant user access to a process, and this is expected because the deployments on SaaS are configured to use the email address of the user as the username claim.

Based on the info above, the user docs should be updated to mention this.

[ThorbenLindhauer]

I'd argue that the docs above are correct. The term "username" refers to the identifier of the user (see for example the API docs). And in an OIDC setup that can be the value of any JWT claim of the user, so in turn is the email address in SaaS.

So what I suggest to do is:

• In the Identity UI user docs (e.g. here: https://docs.camunda.io/docs/next/components/identity/authorization/), explain what the username is and what it's in SaaS (or link to the place where we explain it)
• Optional: Also on https://docs.camunda.io/docs/next/components/concepts/access-control/authorizations/#available-resources, explain what a username is (or link to that explanation in a central place)