[build] Remove support for JDK 8.

[jonpryor]

Context: 7a32bb9
Context: https://stackoverflow.com/questions/55853220/handling-change-in-newlines-by-xml-transformation-for-cdata-from-java-8-to-java
Context: https://bugs.java.com/bugdatabase/view_bug?bug_id=8223291

Background: Since commit bc5bcf4, two JDKs were required in order to fully build and test Java.Interop: JDK 1.8 and JDK-11. This is because src/Java.Interop requires JDK-11+ to build, while some unit tests required JDK 1.8 to pass (because Java XML output changed between JDK 1.8 and JDK-11; see also 7a32bb9).

Recently, a question arose: how well does .NET Android work with JDK 17? (Android Studio recently bumped the bundled JDK from JDK-11 to JDK-17.) The "straightforward" approach of "provision JDK-17 and just build everything with JDK-17" quickly meant that Java.Interop needed to build under JDK-17.

This in turn segued into a "how do I make the MSBuild property meanings clearer", as $(JavaCPath) would be for JDK 1.8, while $(JavaC11Path) was for JDK-11, but with JDK-17 being provisioned $(JavaC11Path) actually was for JDK-17, which is just confusing.

After discussion, we decided that we don't need to continue using JDK 1.8 anymore. Android API-31 requires JDK-11 in order to use various Android SDK build tools, and the Google Play Store requires a target SDK version of API-33 starting 2023-Aug. There is not much point in maintaining JDK 1.8 support.

JDK 11 or later is now required.

Update to use Gradle 8.1.1. This is needed for later JDK-17 support.

java/util/Collection.java existed to help test API documentation import (when $ANDROID_SDK_PATH is set). JDK-11 does not support compiling java/util/Collection.java anymore; it errors out with:

java/java/util/Collection.java:1: error: package exists in another module: java.base
package java.util;
^

"Rename" this type to android/animation/TypeEvaluator.java, and update the API documentation import tests accordingly.

Update the ExpectedTypeDeclaration.MajorVersion values to 0x37.

The .class files for nested types has seen the addition of a new NestHost constant; see also:

• https://docs.oracle.com/javase/specs/jvms/se11/html/jvms-4.html#jvms-4.7.28
• https://docs.oracle.com/javase/specs/jvms/se11/html/jvms-5.html#jvms-5.4.4

Update ExpectedTypeDeclaration.ConstantPoolCount as appropriate.

Additional NestHost-related fallout is that JavaType.class now includes com/xamarin/JavaType$RNC$RPNC in the InnerClasses table.

Update tools/java-source-utils unit tests so that JDK-11 can now be used to run (and pass!) the unit tests. (This previously required JDK 1.8.)

[jpobst]

Should we run a test XA bump against this?

[jonpryor]

@jpobst : we did: dotnet/android#8092

Everything was green except the "linux tests", which suffered bizarro network issues.