Update links to Spring Security's reference documentation

[rwinch]

The docs group has made some changes to how the URLs are going to be handled. Spring Boot should update spring-security attribute to point to https://spring.io/projects/spring-security/reference

[philwebb]

Is it live yet? I get a 404 currently.

[wilkinsona]

As discussed previously, it'd also be good to have a versioned URL so that someone reading a certain version of Boot's docs is linked to the matching version of Security's docs.

[rwinch]

Is it live yet? I get a 404 currently.

Sorry it I pointed to the incorrect URL. It should be http://docs.spring.io/spring-security/reference/

As discussed previously, it'd also be good to have a versioned URL so that someone reading a certain version of Boot's docs is linked to the matching version of Security's docs.

The URL for a specific version is http://docs.spring.io/spring-security/reference/{version}

For example, http://docs.spring.io/spring-security/reference/5.6.0-RC1 is the link for 5.6.0-RC1. For now 5.6.0-RC1 is the latest release (there are no GA releases either), so it redirects to the versionless URL. The URL will stop redirecting once there is a newer version available. Similarly, http://docs.spring.io/spring-security/reference/5.6 will always point to the latest 5.6.x release. For now it redirects to the versionless URL, but once there is a newer GA version available 5.6 will stop redirecting to the versionless URL and point to the latest patch release. This logic is very similar to the symlinks we currently use, but uses redirects instead.

[wilkinsona]

Thanks, Rob.

We currently take the actual Security version and then remove the last . and anything that follows it:

spring-boot/spring-boot-project/spring-boot-docs/build.gradle

Lines 259 to 260 in 313470a

	def securityVersion = versionConstraints["org.springframework.security:spring-security-core"]
	securityVersion = securityVersion.substring(0, securityVersion.lastIndexOf("."))

I think that'll continue to work although I wonder if it's exactly what we want. https://docs.spring.io/spring-security/reference/5.6.0-SNAPSHOT and https://docs.spring.io/spring-security/reference/5.6.1-SNAPSHOT are not found so it looks like we have to do something for -SNAPSHOT versions, but it also sounds like we could leave other versions (5.6.0-RC1, 5.6.0, 5.6.1, and so on) unchanged and they would work.