Use g_memdup2 instead of the deprecated g_memdup

[timopollmeier]

What:

Why:
To avoid warnings about the use of a deprecated function.

How did you test it:

Checklist:

• Tests
• PR merge commit message adjusted

[y0urself]

g_memdup2 is available since glib 2.68, but the Checks run with 2.66 ...
https://github.com/greenbone/gvmd/runs/6402681362?check_suite_focus=true#step:4:36

[y0urself]

I wonder how this works in scanner etc.
https://packages.debian.org/source/bullseye/glib2.0 is only available with 2.66
but according to https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1927 this should be backported to 2.66?

[timopollmeier]

g_memdup2 is available since glib 2.68, but the Checks run with 2.66 ... https://github.com/greenbone/gvmd/runs/6402681362?check_suite_focus=true#step:4:36

I've added a version check so it should work with older versions.