Exclude Xamarin Client from Nuget Updates

[romanett]

Proposed changes

Xamarin Client is not compatible with newer UA .NET Standard OPC Packages therefore try to exclude it form dependabot action (not shure if this really works, seems quite hacky to me).

Related Issues

• Fixes #

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Enhancement (non-breaking change which adds functionality)
• [ ] Test enhancement (non-breaking change to increase test coverage)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

• [ ] I have read the CONTRIBUTING doc.
• [ ] I have signed the CLA.
• [ ] I ran tests locally with my changes, all passed.
• [ ] I fixed all failing tests in the CI pipelines.
• [ ] I fixed all introduced issues with CodeQL and LGTM.
• [ ] I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• [ ] I have added necessary documentation (if appropriate).
• [ ] Any dependent changes have been merged and published in downstream modules.

Further comments

[romanett]

not needed, as .netstandard 2.0 comes back with next nuget version