lib: adapt plugin to use new Sysrepo version

lib: adapt plugin to use new Sysrepo version

Sysrepo recently underwent a complete rewrite, where some substantial
architectural changes were made (the most important one being the
extinction of the sysrepod daemon). While most of the existing API
was preserved, quite a few backward-incompatible changes [1] were
introduced (mostly simplifications). This commit adapts our sysrepo
northbound plugin to those API changes in order for it to be compatible
with the latest Sysrepo version.

Additional notes:
* The old Sysrepo version is EOL and not supported anymore.
* The new Sysrepo version requires libyang 1.x.

Closes #6936

[1] https://github.com/sysrepo/sysrepo/blob/devel/CHANGES

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>