Updated to .Net 8

[NikoMix]

This Pull Request is updating all projects to target .net 8

However, it's dependant on https://github.com/statiqdev/Statiq.Framework/pull/278 and https://github.com/phmonte/Buildalyzer to release a new version (due to issues in resolving legacy dependencies).

Buildalyzer has already changes merged, but not yet released a version with a fix. For performing the update, I used a local reference - once the version is released I'll reverse that change, until then it's just a Draft Pull Request.

In addition Microsoft.Azure.Search had been deprecated (used bei Statiq.Web.Azure) and should be replaced by Azure.Search.Documents. Will migrate this, but did not yet have the chance to setup a test instance.

[CLAassistant]

All committers have signed the CLA.

[daveaglick]

Integrated into #1024 and merged there. Thanks for the groundwork on this! Hopefully I have a 1.0 Statiq release out by the end of the week.