Closes gh-21744

Closes gh-21743

Closes gh-21737

See gh-21739

See gh-21738

See gh-21729

* pr/20929:
  Polish 'Allow beans without public constructors to load'
  Allow beans without public constructors to load

Closes gh-20929

See gh-20929

Allow `BeanDefinitionLoader` to load classes that don't have public
constructors. The constraint was first introduced in d82c5080 to
solve an issue with anonymous Groovy classes but causes particular
problems with `@SpringBootTest`.

See gh-20929

* pr/21732:
  Polish

Closes gh-21732

See gh-21732

See gh-21731

See gh-21730

See gh-21728

Fix the default implementation of `Launcher.isExploded` which should
have returned `true`.

Fixes gh-21575

* pr/21523:
  Polish 'Optimize SystemEnvironmentPropertyMapper'
  Optimize SystemEnvironmentPropertyMapper
  Call append only when necessary
  Use chars rather than strings

Closes gh-21523

Introduce a new `ConfigurationPropertyName.ofIfValid` method to
save us needing to throw and catch an exception unnecessarily.

See gh-21523

See gh-21523

See gh-21523

Update `SystemEnvironmentPropertyMapper` to use single chars
rather than strings whenever possible.

See gh-21523

Fix the regex so that merges to master are supported.

Update `LaunchedURLClassLoader` so that packages defined from exploded
archive folders have manifest attributes applied to them. Prior to this
calling `package.getImplementationTitle()` would only return the a
manifiest attribute when running non-exploded.

The root cause of this issue is the way that `URLClassLoader` handles
the different URL types. For URLs that reference a jar the manifest is
available. For URLs that reference a folder it isn't. When running
exploded we use a URL that references to the `BOOT-INF/classes` folder
directly. To fix the issue we now attempt to detect when `definePackage`
is being called directly, and replace `null` entries with actual
manifest values.

Fixes gh-21705

Closes gh-21720

* pr/21671:
  Remove usages of deprecated Connector.setAttribute()

Closes gh-21671

See gh-21671

* pr/21692:
  Fix event name in documentation

Closes gh-21692

See gh-21692

* pr/21653:
  Polish "Add reference to rocketmq-spring-boot-starter"
  Add reference to rocketmq-spring-boot-starter

Closes gh-21653

See gh-21653