Remove support for .NET 6

[bart-vmware]

Description

This PR removes multi-targeting against .NET 6. This means that from now on, Steeltoe requires .NET 8 or higher.
Aside from that, several small refactorings were applied. See the commit messages for details.

Closes #1249
Closes #1123

Quality checklist

• Your code complies with our Coding Style.
• You've updated unit and/or integration tests for your change, where applicable.
• You've updated documentation for your change, where applicable.
  If your change affects other repositories, such as Documentation, Samples and/or MainSite, add linked PRs here.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.
• You've added required license files and/or file headers (explaining where the code came from with proper attribution), where code is copied from StackOverflow, a blog, or OSS.

[bart-vmware]

/azp run Steeltoe.All

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[bart-vmware]

/azp run cleanup-code

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[azure-pipelines]

Code cleanup successfully reformatted files, but there were no changes to push.

[TimHess]

LGTM