DYN-6421: Update nuspec files for net8

[sm6srw]

Purpose

This pull request does:

• update nuspec files to target the framework specified in the build.xml file.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB

Release Notes

All nuspec files now targets the framework specified in the build.xml file.

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