Support linux older than glibc2.29

[colaub]

On linux lincimgui.so is built against glibc2.29.

Log from a RHEL8 / Rocky8 which based on glibc2.28 :

/lib64/libm.so.6: version `GLIBC_2.29' not found (required by [...]/bin/Debug/net8.0/runtimes/linux-x64/native/libcimgui.so)

rhel8/rocky8 are long-term distributions, that's why it could be interesting to have another linux version in the nuget package. Is it an option?

Thank you for considering the question.

Colin.