Fix outdated label references and links

[Kissaki]

The labels good-first-issue and up-for-grabs no longer exist. Instead, good first issue and help wanted exist.

Live link: https://learn.microsoft.com/en-us/contribute/content/dotnet/dotnet-contribute

[Kissaki]

#sign-off

[learn-build-service-prod]

Learn Build status updates of commit b6be510:

✅ Validation status: passed

File	Status	Preview URL	Details
Contribute/content/dotnet/dotnet-contribute.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[gewarren]

Thanks @Kissaki!

[learn-build-service-prod]

Learn Build status updates of commit 81557df:

✅ Validation status: passed

File	Status	Preview URL	Details
Contribute/content/dotnet/dotnet-contribute.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit d23641a:

✅ Validation status: passed

File	Status	Preview URL	Details
Contribute/content/dotnet/dotnet-contribute.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel