fix!: Change default output directory to build/reports/dependency-check to avoid clash with other reports

[chadlwilson]

• fixes @OutputDirectory conflicts with other plug-ins #482

Note that this is a mildly breaking change so might want to consider that, although if people are sensitive to the location, you'd hope they'd configure it specifically. Technically 12.1.8 was also a mildly breaking change when the directory got marked as an @OutputDirectory as it made Gradle explicitly aware of the ordering problems between tasks, rather than discovering it later.

Setting to the current build/reports directory tends to break task caching for other plugins since many of them put things into /reports including Gradle's reports/problems/problems-report.html, test reports into reports/tests. When ODC writes things there, Gradle isn't sure what to do as it implies ordering issues between tasks, either causing tasks to unnecessarily re-run, or users to need to define dependencies between tasks, which is quite confusing.

It requires a fair bit of Gradle experience to figure out that the ODC directory needs to be changed to work around it.

• I considered declaring a set of @OutputFIles instead (which would be backward compatible) but that's not so safe/easy to do with the interface to ODC.
• Changed @InputDirectory -> @Input. @InputDirectory is for directories whose contents you care about, and where it should cause the task to re-run if cached. In this case we only care about the path, not the contents.

TODO afterwards

• Update website documentation

[jeremylong]

I completely agree with the change - but this could be considered breaking by some. We do have plans for other breaking changes to be released soon. I'll include this then.