Add Qt Major Version in lib soname

[sfalken]

As currently setup, whether you build stellarsolver against Qt5 or Qt6, the SONAME of libstellarsolver is remaining the same:

📦 mustang /usr/lib64 % objdump -p libstellarsolver.so.2.6 | grep SONAME
  SONAME               libstellarsolver.so.2

this makes the Qt5 and Qt6 versions of stellarsolver be incompatible with each other, and not co-installable, which is a problem for downstream packaging of the project.

Can we get the Qt Major Version added to the library SONAME?