C# GenericHost Support multi targetting

[EraYaN]

Split 3 of #22062

This makes it so that when you have both net8 and net9 (for example) the csproj includes both. This makes sure that consumers of your package will get to use the newest transitive dependencies.

This requires a bunch of Condition clauses.

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[wing328]

@EraYaN when you've time, can you please PM me via Slack to quickly go through your PRs and get them merged? Thank you.

https://join.slack.com/t/openapi-generator/shared_invite/zt-36ucx4ybl-jYrN6euoYn6zxXNZdldoZA

[EraYaN]

I have updated this branch (recreated the changed really) and it seems that this PR also carries some of the sample updates from #22232 even though those all seem to already be on the master branch.

EDIT: One more rebase fixed it.