* pr/24625:
  Update copyrights to 2021

Closes gh-24625

See gh-24625

See gh-24604

Closes gh-24599

Closes gh-24596

Closes gh-24586

See gh-24453

* pr/24558:
  Polish

Closes gh-24558

See gh-24558

See gh-24205

Closes gh-24567

Closes gh-24205

Closes gh-24531

Closes gh-24560

See gh-18631

Rename "app" projects in `spring-boot-launch-script-tests` and
`spring-boot-loader-tests` to something unique.

See gh-18631

Update the jar `Handler` class to support a non-reflective fallback
mechanism when possible. The updated code attempts to capture a regular
jar URL before our handler is installed. It can then use that URL as
context when creating the a fallback URL. The JDK jar `Handler` will
be copied from the context URL to the fallback URL.

Without this commit, resolving new Tomcat URLs of the form
`jar:war:file:...` would result in an ugly "Illegal reflective access"
warning.

Fixes gh-18631

See gh-11987

Fixes gh-11987

Fixes gh-24522

* pr/24532:
  Avoid duplicates in MergedContextConfiguration

Closes gh-24532

Remove duplicate config attribute classes by using a `Set` rather
than a `List` in the `SpringBootTestContextBootstrapper`.

See gh-24532

Prior to this commit, running the bootBuildImage Gradle task on a
project configured for war packaging would result in a jar file being
built and used in the image instead of the war file. With this commit
an error will be thrown from the plugin in this case.

Fixes gh-24521

Closes gh-24486

Closes gh-24513

Closes gh-24472

Closes gh-24500

Fixes gh-24475

Closes gh-24494

This commit removes links to Jetty API and documentation as those links
are breaking every time the Jetty team releases a new version

See https://github.com/eclipse/jetty.project/issues/4546

Closes gh-24493

Closes gh-24463

Closes gh-24464

Closes gh-24465

Close gh-24462

Closes gh-24457

Closes gh-24456