docs: update badge

[WeihanLi]

Proposed Changes

Seemed the AppVeyor is not used now? so try to remove AppVeyor badge
Update Github Actions linux build badge with a valid one

Redenered view(https://github.com/WeihanLi/rabbitmq-dotnet-client/tree/patch-1):

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

[lukebakken]

Thanks for noticing!

[lukebakken]

@WeihanLi just FYI, this allows a user to click on a badge to go to the workflow:

ae88255