Move to the snapshots and rework changed APIs in preparation for
the upcoming rc8 release.

See gh-11575

* pr/11792:
  Polish

Closes gh-11792

* gh-10653:
  Migrate spring-boot-sample-web-ui to Bootstrap v4

Closes gh-10653

Closes gh-11768

Closes gh-11755

* gh-11784:
  Polish "Mark thread as interrupted after catching InterruptedException"
  Mark thread as interrupted after catching InterruptedException

See gh-11784

Closes gh-11784

* gh-11751:
  Polish "Use custom DataSource if Flyway or Liquibase has user or url"
  Use custom DataSource if Flyway or Liquibase has user or url

Closes gh-11751

This commit enables a more flexible Liquibase/Flyway configuration by
allowing for a combination of the provider's and the primary
DataSource's configuration to be used. This gives developers the
flexibility to specify only a user or a url and having
Liquibase/Flyway fall back to individual datasource properties rather
than ignoring the Liquibase/Flyway properties and falling back to the
default data source.

See gh-11751

* pr/11772:
  Polish "Migrate server customizer to PropertyMapper"
  Migrate server customizer to PropertyMapper

Closes gh-11772

See gh-11772

Closes gh-11755

Closes gh-11788

* pr/11789:
  Align missed bits from trace endpoint changes

Closes gh-11789

See gh-11537

* pr/11773:
  Polish "Use PropertyMapper to configure WebServerFactory"
  Use PropertyMapper to configure WebServerFactory

Closes gh-11773

See gh-11773

Add an operation on PropertyMapper that takes care of casting. Returns
a source for the requested type if the current value is of the right
type.

Closes gh-11788

Fixes gh-10531

* pr/11783:
  Polish contribution
  Polish

Closes gh-11783

See gh-11783

* pr/11785:
  Remove unused parameters

Closes gh-11785

Closes gh-11745

* pr/11775:
  Upgrade to Maven Checkstyle Plugin 3.0.0

Closes gh-11775

Closes gh-8933

There's an extra ClassLoader in the hierarchy of the TCCL in Groovy
2.4 vs Groovy 2.5 so we require an extra getParent() call to avoid
being able to load classes that are visible to the launched URL
class loader.

Closes gh-11745