[Tracking] Improve Android API compat test in POM

[jhy]

Currently in the POM we have this:

jsoup/pom.xml

Lines 115 to 155 in 19c758e

	<execution>
	<id>check-android21-api</id>
	<phase>compile</phase>
	<goals>
	<goal>check</goal>
	</goals>
	<configuration>
	<signature>
	<groupId>com.toasttab.android</groupId>
	<artifactId>gummy-bears-api-21</artifactId>
	<version>0.10.0</version>
	<!-- <classifier>coreLib2</classifier> -->
	<!-- ^^ https://github.com/open-toast/gummy-bears says coreLib2 classifier for desugar should work, but I can't seem to wrangle Animal Sniffer to support that, so reverting to ignores for desugar -->
	</signature>
	<ignores>
	<ignore>java.io.File</ignore> <!-- File#toPath() -->
	<ignore>java.nio.file.*</ignore>
	<ignore>java.nio.channels.SeekableByteChannel</ignore>
	<ignore>java.util.function.*</ignore>
	<ignore>java.util.stream.*</ignore>
	<ignore>java.lang.ThreadLocal</ignore>
	<ignore>java.io.UncheckedIOException</ignore>
	<ignore>java.util.Comparator</ignore> <!-- Comparator.comparingInt() -->
	<ignore>java.util.List</ignore> <!-- List#stream() -->
	<ignore>java.util.ArrayList</ignore> <!-- List / ArrayList #sort() -->
	<ignore>java.util.LinkedHashMap</ignore> <!-- LinkedHashMap#computeIfAbsent() -->
	<ignore>java.util.Map</ignore> <!-- Map#computeIfAbsent() -->
	<ignore>java.util.Objects</ignore>
	<ignore>java.util.Optional</ignore>
	<ignore>java.util.Set</ignore> <!-- Set#stream() -->
	<ignore>java.util.Spliterator</ignore>
	<ignore>java.util.Spliterators</ignore>
	<ignore>java.nio.ByteBuffer</ignore> <!-- .flip(); added in API1; possibly due to .flip previously returning Buffer, later ByteBuffer; return unused -->
	<ignore>java.net.HttpURLConnection</ignore><!-- .setAuthenticator(java.net.Authenticator) in Java 9; only used in multirelease 9+ version -->
	<!-- HttpClient and following in Java 11; only used in multirelease 11+ version, guarded and not on Android -->
	<ignore>java.net.http.*</ignore>
	<ignore>java.time.Duration</ignore>
	<ignore>java.util.OptionalLong</ignore>
	</ignores>
	<!-- ^ Provided by https://developer.android.com/studio/write/java8-support#library-desugaring -->
	</configuration>

Because the Maven Animal Sniffer plugin doesn't currently support a classifier (mojohaus/animal-sniffer#328), we can't specify the Gummy Bears API definition for coreLib2. So I've been updating the POM with manual exclusions. But that's dull and a bit risky.

So I've added a PR to implement the classifier support: mojohaus/animal-sniffer#346

Assuming that lands we should be able to simplify this POM quite a bit.

It will be good also if Gummy Bears has a coreLib2 + NIO definition set, because that's what jsoup aligns to.

[jhy]

I have also raised a PR in Gummy Bears for NIO (and also the minimal flavor, for completedness of the feature): open-toast/gummy-bears#220

[jhy]

Once these are hopefully merged, the POM can be substantially simplified: master...validate-corelib2-nio