This commit fixes the URI which is constructed when only a version is set in the
metadata repository. Before this change, whatever version was used in the configuration,
we would ignore it, because we were testing against the wrong URI to figure out if the
user had configured something explicitly or not.

Now, we will prioritize the user URI, then construct a GitHub release URI from the
version which is configured, and eventually use Maven Central if that URI is the
same as the default one.

Fixes #424

Create release 0.9.21

Fix lookup of metadata repository

This new index.json attribute is design to allow more
flexible version matching to avoid breakage due to
switch on latest version for untested ones.

It should have a `default-for` key and a regexp as a
string value, for example "1\\.0\\..*" which should
match versions like "1.0.0", "1.0.1", "1.0.2", etc.

See oracle/graalvm-reachability-metadata#62
See oracle/graalvm-reachability-metadata#275

Perform 2 distinct findConfigurationFor invocations
in order to not be dependent of the order.

Add support for `default-for` attribute

….3.0

Bump metadata version to 0.3.0