Preparing for GWT 2.13 release

[Colin Alworth]

I'm sorry for the delay here, but I think we're just about ready to plan a release. We currently have three PRs pending for 2.13 - https://github.com/gwtproject/gwt/milestone/23:

• https://github.com/gwtproject/gwt/pull/10249 makes xml-apis optional when using any GWT classes at compile or runtime. Setting the system property "gwt.validation.ignoreXml" to "true" will not look for xml config files, so xml-apis/jaxb are no longer needed. 
• https://github.com/gwtproject/gwt/pull/10247 is a draft (needs documentation and 10249) to update samples to use Maven rather than Ant and to be multi-module where appropriate. 
• https://github.com/gwtproject/gwt/pull/10208 gives us a new JRE emulation documentation page, outlining missing classes/members in currently-supported packages, and providing links to issues documenting their status (impossible, infeasible, patches welcome, triage)

Some highlights since 2.12.2:

• Removed more IE checks/support
• Remove GWT polyfills for features that all modern browsers support
• Improve context in various compiler errors
• Also add JFR events to assist in debugging/profiling the compiler, and enhance the "ast dump" feature for tracing generated output
• Update JDT so we can consume JDT from Maven Central rather than a custom build, fix a few related bugs
• Lots of JRE emulation enhancements
• Deprecate and stop using some "internal-ish" APIs such as Util, Utilities
• Improve compiled output size in a few specific cases
• Disable Jetty Servlet support in DevMode by default, to be removed in a future release.
• Replace GWT SDK-specific help text with links to gwtproject.org

In theory the server should continue to work on Java 8, though this isn't extensively tested - if you're interested in this, please join our testing efforts.

Java 11 is still the minimum for any dev tools, including the compiler, test runner, dev mode, codeserver. Next release is likely to bump this up to Java 17 to allow us to further update JDT (and possibly our internal Jetty as well).

Once we have these last few commits merged, we'll be looking for testers. Today, you can either downloads a GWT SDK zip from github, or the HEAD-SNAPSHOT version from https://central.sonatype.com/repository/maven-snapshots/, the sonatype snapshot repository. Please reply to this email (or to me personally) if you're able to assist with testing, including the OS(s), browser(s) and Java version(s) you have available to you.