Closes gh-23409

Closes gh-23408

Previously, spring.flyway.init-sqls was only applied to Flyway's
configuration if Flyway was being configured to create the DataSource.
If Flyway was being configured to use an existing DataSource, init-sqls
was not applied. This is a hangover from when the init SQLs support was
introduced. At that time, Flyway only supported SQL to initialize the
connection when it was creating the DataSource. Flyway 5.2 added init
SQL support no matter how Flyway's DataSource was configured.

This commit updates FlywayAutoConfiguration to always apply the
init-sqls property to Flyway's configuration. The property's
documentation does not describe the current limitation so this change
should align the behaviour with what the documentation leads people to
expect.

Fixes gh-23392

Closes gh-23407

Closes gh-23406

Fixes gh-23402

Update `ConfigDataEnvironment` so that it adds the initial `Binder`
to the `BootstrapContext` for `Bootstrappers` to use.

Closes gh-23401

Refine `OriginTrackedPropertiesLoader` document split detection to be
more lenient if there is trailing whitespace.

Closes gh-23399

* gh-23391:
  Upgrade Java 15 CI image to use the Java 15 GA build

Closes gh-23391

See gh-23391

Closes gh-23397

Closes gh-23396

Closes gh-23395

Closes gh-23394

Closes gh-23393

See gh-23326

Attempt to fix `ConcurrentModificationException` which occurs on
Java 11+.

See gh-23326

Refactor `BootstrapRegistry` support following initial prototype work
with the Spring Cloud team.

This update splits the `BootstrapRegistry` API into `BootstrapRegistry`,
`BootstrapContext` and  `ConfigurableBootstrapContext` interfaces and
moves it to the same package as `SpringApplication`.

A new `Bootstrapper` interface has been introduced that can be added
to the `SpringApplication` to customize the `BootstrapRegistry` before
it's used.

Closes gh-23326

Move `IncompatibleConfigurationFailureAnalyzer` to fix a package tangle.

Closes gh-23385

Relocate `DefaultPropertiesPropertySource` to fix a package tangle.

Closes gh-23383

Closes gh-23379

Closes gh-23188

Closes gh-23187