CMake: add TCL/Tk 8.7 as supported versions

[fluffy]

Summary

Since TCL/Tk 8.7 release hits the ports and allowed to be default versions, teach CMake modules to find its right way.

[tcberner]

This should be sent upstream too.

[fluffy]

In D16320#346388, @tcberner wrote:

This should be sent upstream too.

submitted as https://gitlab.kitware.com/cmake/cmake/issues/18186

[tcberner]

Thanks :)