Update the Maven and Gradle documentation following the refined
layer customization changes.

See gh-20526

Update the reference documentation following the jar format changes.

See gh-20813

Update layertools to support the flat jar format. Layers are now
determined by reading the `layers.idx` file.

Closes gh-20813

Update the Gralde plugin so that layered jars now use the regular "flat"
format. The layers.idx file now describes which layer each file should
be placed.

See gh-20813
Co-authored-by: Phillip Webb <pwebb@pivotal.io>

Update the Maven plugin so that layered jars now use the regular "flat"
format. The layers.idx file now describes which layer each file should
be placed.

See gh-20813
Co-authored-by: Phillip Webb <pwebb@pivotal.io>

Create a dedicated layer that is used to hold the launcher support
classes. The layer sits between `dependencies` and
`snapshot-dependencies` so that the layer is sensible for both
SNAPSHOT and RELEASE versions of Spring Boot

Closes gh-20529

Simplify layer customization logic for both Maven and Gradle and
refactor some internals of the Gradle plugin.

Both Maven and Gradle now use a simpler customization format that
consists of `application`, `dependencies` and `layer order` sections.
The `application`, `dependencies` configurations support one or more
`into` blocks that are used to select content for a specific layer.

Closes gh-20526

Update `spring-boot-jarmode-layertools` so that it no longer required
a `spring-boot` jar to run.

Closes gh-20815

Closes gh-19426

Closes gh-20584

This commit adds the new "`spring.main.cloud-platform`" configuration
property. This allows applications to override the auto-detection and
force a specific Cloud Platform. This is useful for testing behavior on
a local machine or force the detection of a particular platform.

This commit also adds a new `CloudPlatform.NONE` value that allows
applications to disable the auto-detection of the Cloud Platform, thus
avoiding issues with false positives.

Closes gh-20553

Closes gh-16998

* wonwoo/master:
  Polish
  Configure codec buffer size in ES Reactive Rest client

Closes gh-20205

See gh-20205

This commit adds a new configuration property
`"spring.data.elasticsearch.client.reactive.max-in-memory-size"`
which configures the maximum amount of memory buffered by the
`WebClient` used by the Reactive ElasticSearch client.

See gh-20205

Closes gh-20808

Closes gh-20807

Closes gh-20804

Closes gh-20803

* pr/20756:
  Upgrade to Docker 19.03.8 in CI

Closes gh-20756

See gh-20756

Closes gh-20802

Closes gh-20801

* pr/20793:
  Harmonize issue title for auto-created Docker upgrade issues

Closes gh-20793

See gh-20793

This commit adds support for Cloud Native Buildpacks builder platform
API 0.3, which is the latest platform API available currently. Support
for platform API 0.1 has been removed, adopting the policy of the pack
CLI to support the current platform API version and one version prior.

Fixes gh-20757

CNB builder images are now being tagged in a manner that indicates
the version of the platform API implemented. This allows Spring Boot
to default to a builder tag that guarantees API compatibility while
allowing for updates to bundled buildpacks.

Fixes gh-20171

Restructure and polish docker code to fix a package tangle and
provide better separation of concerns.

Closes gh-20797

* pr/20729:
  Polish "Disable Hazelcast auto-configuration when Jet is present"
  Disable Hazelcast auto-configuration when Jet is present

Closes gh-20729