Update samples

[mattjohnsonpint]

• Removes the "AspNetCore5.Mvc" sample, as it's already covered by the "AspNetCore.Mvc" sample
• Update all samples to current targets
• Update all samples to current versions of external dependencies
• Update code where needed to be compatible with updates

Probably still need to do a more thorough pass on each sample, or revise them completely. But this at leasts brings the code current.

#skip-changelog