While builds using maven are relatively likely to target jdk8,
they typically support building using default jdk.
The main exception is the default jdk no longer supports targeting
JRE's before version 8. For projects that do that we can override the
jdk version for the maven used there.
Uses the HTTPS url for cases where the existing URL has a permanent
redirect. For each domain, at least one fixed derivation URL was
downloaded to test the domain is properly serving downloads.
Also fixes jbake source URL, which was broken.
