Fix POMs following project relocations and apply CI friendly Maven
conventions.

See gh-9316

Move integration and deployment tests under a single `spring-boot-tests`
module.

See gh-9316

Move projects to better reflect the way that Spring Boot is released.

The following projects are under `spring-boot-project`:

  - `spring-boot`
  - `spring-boot-autoconfigure`
  - `spring-boot-tools`
  - `spring-boot-starters`
  - `spring-boot-actuator`
  - `spring-boot-actuator-autoconfigure`
  - `spring-boot-test`
  - `spring-boot-test-autoconfigure`
  - `spring-boot-devtools`
  - `spring-boot-cli`
  - `spring-boot-docs`

See gh-9316

Ignore `secrets.yml` so that if can be used to hold sensitive
credentials.

See gh-9316

Ignore flattened POM files which will be generated once the build has
been reworked.

See gh-9316

Closes gh-10286

Update `LettuceConnectionConfiguration` so that `commons-pool2` can
be an optional dependency.

See gh-10480

This commit changes the default client to Lettuce. This has the side
effect of making the `spring-boot-starter-redis-reactive` irrelevant.

Recent improvements made pooling optional so `commons-pool2` is no
longer provided by default either.

Closes gh-10480

* pr/10508:
  Upgrade to HikariCP 2.7.2

Closes gh-10508

* pr/10504:
  Upgrade to JUnit Jupiter 5.0.1

Closes gh-10504

Update `spring-boot-loader` to use the same dependency versions as the
main project.

Closes gh-10446

Fixes gh-10505

Closes gh-10319

This commit clarifies that DataSourceInitializedEvent is only to be
used by the datasource initializer facility and JPA (Hibernate). The
even is renamed to DataSourceSchemaCreatedEvent to clarify what it
actually signals.

Closes gh-4292

This commit clarifies the scope of the datasource initializr. In
particular, it is not possible to create the schema with that facility
and let Hibernate creates additional tables.

Closes gh-9048

This commit changes HibernateJpaAutoConfiguration to back off if no
single datasource candidate is available in the context.

Closes gh-5541

This commit separates the lifecycle of the datasource initialization
from DataSourceInitializer itself. It also makes sure that a @Primary
data source is no longer required.

Closes gh-10502

Closes gh-10501

* pr/10498:
  Upgrade to Maven Compiler Plugin 3.7.0

Closes gh-10498

Closes gh-10497

* gh-10495:
  Remove BeansEndpoint.BeansEndpointResponse

Closes gh-10495

Previously, if the file watcher thread tried to stop the
FileSystemWatcher when another thread was already stopping it a
livelock could occur. The livelock occurred because the file watcher
thread would attempt to lock a monitor that was being held by a thread
that had joined the file watcher thread and was waiting for it to die.

This commit avoid the livelock by narrowing the synchronization that's
used when stopping the FileSystemWatcher. The monitor is used to
obtain a reference to the file watcher thread in a thread-safe manner,
but it is released prior to joining the file watcher thread and
waiting for it to die. This will allow a parallel attempt by the
file watcher thread to stop itself to succeed.

Closes gh-10496

Closes gh-10492

Closes gh-10491

Closes gh-10490