Documentation should point to spring-security-samples

[jzheaux]

In 5.5, gh-samples-url points to https://github.com/spring-projects/spring-security/tree/5.5.0/samples, but the samples have moved to spring-security-samples, so the links are broken.

The documentation should be updated in the following way:

• Add messaging to documentation about sample migration Add messaging to documentation about sample migration #9815
• Update links to point to migrated samples Update links to point to migrated samples #9816
• Remove links that point to unmigrated samples Remove links that point to unmigrated samples #9817
• Consider adding a link checker to build Consider adding a link checker to build #9818
• Include samples in build Verify Samples in Build #9846
• Fix broken links in docs Fix broken links in docs #9869

[marcusdacoregio]

@jzheaux do you think that we can close this issue? I've opened ticket the spring-projects/spring-security-samples#22 to map the examples left to migrate