Adds support for net10.0

[kailash-b]

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Description

This PR adds support for .NET 10.0 across the library, build workflows, and documentation. The following updates were made:

• Updated .github/workflows/build.yml and .github/workflows/rl-secure.yml to include net10.0 in the build matrix and setup steps.
• Updated documentation in README.md and docs-source/index.md to state support for ".NET 6.0 and above"
• Updated Auth0.AspNetCore.Authentication.csproj and playground/test projects to target net10.0.
• Updated references to include net10.0 compatible references
• Improved error response serialization in BackchannelLogoutHandler.cs for compatibility.
• Enhanced integration tests to use a new wellknownconfig_with_par.json resource, and updated test logic to reference this file for PAR endpoint scenarios.

References

• SDK-7170

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not main