Explanation of start of release changes

## Overview

The start of release changes, the changes that turn JDK N into JDK (N+1), are primarily small updates to various files along with new files to store symbol information to allow `javac --release N ...` to run on JDK (N+1).

The updates include changes to files holding meta-data about the release, files under the `src` directory for API and tooling updates, and incidental updates under the `test` directory.

## Details and file updates

As a matter of policy, there are a number of semantically distinct concepts which get incremented separately at the start of a new release:

- Feature value of `Runtime.version()`
- Highest source version modeled by `javax.lang.model.SourceVersion`
- Highest class file format major version recognized by the platform
- Highest `-source`/`-target`/`--release` argument recognized by `javac` and related tools

The expected file updates are listed below. Additional files may need to be updated for a particular release.

### Meta-data files

- `jcheck/conf`: update meta-data used by `jcheck` and the Skara tooling
- `make/conf/version-numbers.conf`: update to meta-data used in the build

### `src` files

- `src/hotspot/share/classfile/classFileParser.cpp`: add a `#define` for the new version
- `src/java.base/share/classes/java/lang/classfile/ClassFile.java`: add a constant for the new class file format version
- `src/java.base/share/classes/java/lang/reflect/ClassFileFormatVersion.java`: add an `enum` constant for the new class file format version
- `src/java.compiler/share/classes/javax/lang/model/SourceVersion.java`: add an `enum` constant for the new source version
- `src/java.compiler/share/classes/javax/lang/model/util/*` visitors: Update `@SupportedSourceVersion` annotations to latest value. Note this update is done in lieu of introducing another set of visitors for each Java SE release.
- `src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java`: add an `enum` constant for the new source version internal to `javac`
- `src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/ClassFile.java`: add an `enum` constant for the new class file format version internal to `javac`
- `src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Target.java`: add an `enum` constant for the new target version internal to `javac`
- `src/jdk.compiler/share/classes/com/sun/tools/javac/processing/PrintingProcessor.java` update printing processor to support the new source version
- The symbol information for `--release` is stored as new text files in the `src/jdk.compiler/share/data/symbols` directory, one file per module. The README file in that directory contains directions on how to create the files.

### `test` files

- `test/langtools/tools/javac/api/TestGetSourceVersions.java`: add new `SourceVersion` constant to test matrix.
- `test/langtools/tools/javac/classfiles/ClassVersionChecker.java`: add new enum constant for the new class file version
- `test/langtools/tools/javac/lib/JavacTestingAbstractProcessor.java` update annotation processor extended by `javac` tests to cover the new source version
- `test/langtools/tools/javac/preview/classReaderTest/Client.nopreview.out` and `test/langtools/tools/javac/preview/classReaderTest/Client.preview.out`: update expected messages for preview errors and warnings