This commit adds an additional 'layers/configuration' property that can
be used to refer to a separate layers configuration file. This separate
file defines:

* The layers and their order of precedence,
* How libraries are handled using filters that match against the
coordinates of each library, and
* How classes are handled using filters that match against the location
of the entry

An XSD to validate the XML configuration file is available.

Closes gh-20295
Co-authored-by: Stephane Nicoll <snicoll@pivotal.io>

Closes gh-20489

Closes gh-20469

* gh-20417:
  Polish "Add clearChecksums to Liquibase auto-configuration"
  Add clearCheckSums to Liquibase auto configuration

Closes gh-20417

See gh-20417

Liquibase auto configuration is extended with clearCheckSums to allow
to clear all checksums in the current changelog, so they will be
recalculated upon the next update.

See gh-20417

Closes gh-20503

Previously, DevToolsDataSourceCondition called
getBeanNamesForType(Class) which could trigger unwanted initialization
of lazy init singletons and objects created by FactoryBeans.

This commit updates DevToolsDataSourceCondition to prohibit eager
init when getting the names of the beans of a particular type.

Fixes gh-20430

Closes gh-20502

Closes gh-20501

Previously, DependencyCustomizer's ifAllResourcesPresent method
would return a customizer that added modules if any of the resources
were present. This commit corrects this behaviours so that modules are
only added if all resources are present.

Fixes gh-20418

* pr/20498:
  Polish

Closes gh-20498

See gh-20498

Closes gh-20241

The fix was applied as part of 308e1d36. This commit merges forwards
the test that was added in 2.1.x and 2.2.x.

Fixes gh-20386 in 2.2.0.M3.

Closes gh-20499

TomcatReactiveWebServerFactory's getTomcatWebServer is described as
being a hook point the can be overrideden to return a different
TomcatWebServer or to apply additional processing to the Tomcat
server. Previously, this did not work as the method was not called
to create the TomcatWebServer instance. Instead, TomcatWebServer was
instantiated and returned directly.

This commit updates TomcatReactiveWebServerFactory to call
getTomcatWebServer to create the TomcatWebServer instance. A test
has also been added to verify that the method is now used.

Fixes gh-20386

Closes gh-20491

* gh-20436:
  Update versiion of git-properties plugin shown in the docs

Closes gh-20436

See gh-20436

Unfortunately, while redundant for new applications, removing the
leading slash adversely affected existing application upon upgrades as
it caused Liquibase to re-apply every change log.

Closes gh-20177

* gh-20472:
  Improve unused exclusion detection and remove those that are unused

Closes gh-20472

See gh-20472

See gh-4657

Closes gh-20488

Closes gh-20487

Closes gh-20486

Closes gh-20485

Closes gh-20484

Closes gh-20441

Closes gh-20482

Closes gh-20481

Closes gh-20480

Closes gh-20479

Closes gh-20477

Closes gh-20476

Closes gh-20475

Closes gh-20474

Closes gh-20473

Closes gh-20483