feat: Jwt -- don't force "upn" claim

[manger]

Description

Fixes #5151

Adjusts the Jwt class so that userPrincipal() returns the value of a upn, preferred_username, or sub claim in that order of preference; but the class does not automatically insert a upn claim whenever a preferred_username or sub is present. This means userPrincipal() returns a value to satisfy the Eclipse MicroProfile Interoperable JWT RBAC. But the Jwt class is still usable for use-cases beyond that very specific profile where a upn claims in not required, not desired, and may even be forbidden.

A upn claim can still be set.

Unit tests are added to confirm that Jwt.userPrincipal() falls back to sub if upn is absent; and to confirm that upn is no longer automatically (and unexpectedly) added just because sub is set.

All the existing unit tests still pass.

Documentation

The javadoc for Jwt.userPrincipal() and Jwt.Builder.userPrincipal(String) is adjusted.

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