5Axis Kinetic from 2.8 on 2.7x

No that won't work. The kinematic has to reflect the axis setup of the machine. But there is a xyzac-trt kinematic in linuxcnc.

linuxcnc.org/docs/2.8/html/motion/5-axis-kinematics.html

If you have a xyzac-trt setup but with a gantry using two motors for the same axis say xyyzac then you will need to adjust the kinematics file and rebuild linuxcnc.


[edit]
This is for changing xyzbc to xyzzbc kinematic but it's the same (fairly straight forward) procedure:

forum.linuxcnc.org/10-advanced-configura...-kins-module?start=0

In which folder is this configuration for tilting spindle found?

Build means, compile?

"Trt" means, tilting table, I need to convert to tilting spindle (srt) Spindle rotating tilting

Ah, I see, -trt won't work for a tilting head setup. As far as I know Bridgemill is the only kinematic where the spindle does both rotations.
So really in your case it's just that x and y axis are swapped compared to the "bridgemill" simulation?

In this document it says that it is in the git tree, but what is "git tree?" is that on the internet? Or in some directory on my computer?