Update links to point to migrated 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.

For links that point to samples that have been migrated, they should point to the tagged version of the corresponding tag and sample in spring-security-samples.

Note that the docs should continue to use variables to compute the links in order to simplify future maintenance.

Since this first released in 5.5, these links should be backported to the 5.5 documentation as well.