This commit updates (again) the URL used by Initializr clients to fetch
metadata for the Spring Boot project.
Since the Sagan application is being retired, this API has now migrated
to https://api.spring.io
See gh-1369
Prior to this commit, the Spring Boot metadata reader would use a legacy
API endpoint on spring.io to fetch the Spring Boot versions.
The current website is about to be retired for a different
infrastructure and we should take this opportunity to update the
endpoint we're using.
This commit updates the metadata reader strategy to now use the
`https://spring.io/api/projects/spring-boot/releases` endpoint to fetch
the relevant information. The JSON format itself changed slightly and
this commit adapts to the new format as well.
Closes gh-1369
