Compile flag support for Unity6

[hugoymh]

Unity UI Extensions - Pull Request

Overview

Propose to start adding support for Unity 6. Version Compile Flags are used

Changes

in CardPopup2D.cs, added compile flags regarding Rigidbody Component's velocity:

#if UNITY_6000_0_OR_NEWER
                rbody.linearVelocity = Vector3.zero;
#else
                rbody.velocity = Vector3.zero;
#endif