Bump to .Net 8 + Update Nuget Packages to non vulnerable version

[NikoMix]

Since I wanted to work some more with Statiq and figured out there had been some issues to integrate it with the current LTS (.net 8) version, started to work on an commit. Only shortly before wrapping up the PR figured out #270 had been open - but already since January, so assuming it's somewhat stale.

A few notes:

• Statis.Tables had not been updated on EPPlus due to a more restrictive license change (pay to use)
• A few breaking changes have been introduced, with the update of some nuget packages
  • Loading of SixLabors Image changed, where format is not an output parameter anymore, instead read through image metadata
  • Asserts have been restructured to use Assert.That() instead extension methods like .True()/.False()/.Equals()/etc.
  • SemVer is now SemanticVersioning
  • RecyclableMemoryStreamManager is instanciated through a options object, instead of parameters and property setters
  • XML Navigation with .Descendents().OfType() is obsolete and use of .Descendants() encouraged (uses the same under the hood)
  • Added few ArgumentNullChecks (where it would not really interrupt any code)
• Quite a lot of new Code Analyzers have been introduced, but for now put as warning to keep the change as non-intrusive as possible (to avoid a super large change)

Builds on my machine, but not sure if/what/how to get the test's run, so a proper validation would still be pending - but compiles for me :) Tried to avoid adding any new or removing any existing code/features.

[NikoMix]

This was an actual mistake by me in translating the "old" asserts into the "new" - so corrected.

[NikoMix]

Technically Struct is actually more correct, but seems that prior frameworks reported SpecificKind as Structure.

[NikoMix]

With more recent Handlebars version, the BlockHelper needs to be started by # - can be seen in the official handlebars repository. ⚠️ CAUTION: This is likely to be a breaking change to prior Statiq.Framework users - but would make sense to follow the "official" handlebars behaviour

[NikoMix]

Applied Globalization on TestFixture, however only two tests had been affected on my local machine by the coma seperator due to german localization.

[NikoMix]

This was a little more tricky - ShouldContain is (now?) clipped to 100 chars and the match is actually in the testdocument content - however all further text is cut-off with "..." as such the test fails. ShouldContainWithoutWhitespace is the only extension method, which is not clipping the actual. Tried to explain through comment, so it's not reversed as it's not very "intuitive"

[NikoMix]

Last issue center around Statiq.Sass, which does not seem to be compatible with ARM64 which seems to be the case for macos runners from GitHub Actions (see open issue here: xoofx/SharpScss#19) a workaround might fix the ability to run on the agent - but would not run on end-users working with Statiq on macos. @daveaglick would you have a take on that?

[daveaglick]

Thanks for all the work on this! Not sure when I'll have a chance to review it, but I'm hoping to carve out time soon since it covers what was the last thing I wanted to do before finally releasing a 1.0 version.

[NikoMix]

Sure, happy to contribute and no pressure. Been looking to make the update for Statiq.Docs and .Web as well - but the Scss issue is making this particular pull request somewhat tricky as it seems either a dirty workaround (installing libscss for the agent on macos only) or replacing SharpScss (?). Both did not seem ideal, if you have any preference/alternative more than happy to take this over the finishing line and you can review at your leisure