Previously, remote DevTools only correctly supported modifying
existing classes. New classes that were added would be missed, and
deleted classes could cause a failure as they would be found by
component scanning but hidden by RestartClassLoader.

This commit introduces a DevTools-specific ResourcePatternResolver
that is installed as the application context's resource loader. This
custom resolver is aware of the files that have been added and
deleted and modifies the result returned from getResource and
getResources accordingly.

New intergration tests have been introduced to verify DevTools'
behaviour. The tests cover four scenarios:

- Adding a new controller
- Removing an existing controller
- Adding a request mapping to a controller
- Removing a request mapping from a controller

These four scenarios are tested with:

- DevTools updating a local application
- DevTools updating a remote application packaged in a jar file
- DevTools updating a remote application that's been exploded

Closes gh-7379

See gh-7357

* pr/7336:
  Polish multiple root contexts in `Restarter`
  Support multiple root contexts in `Restarter`

See gh-7335
See gh-7336

Update devtools restarter to support multiple application contexts.

Fixes gh-7335
Closes gh-7336

* pr/7373:
  Polish content security policy contribution
  Add properties for content security policy

See gh-7373
See gh-7373

Add `content-security-policy` and `content-security-policy-mode`
`security.header` properties and update auto-configuration to apply
them when specified.

Fixes gh-7373
Closes gh-7357

* pr/7393:
  Polish ErrorPageFilter.getDescription javadoc
  Make ErrorPageFilter.getDescription protected

See gh-7393

Make `ErrorPageFilter#getDescription` `protected` instead of `private`
to be able to customize the details for the request logged in case of
an error.

Fixes gh-7380
Closes gh-7393

Fixes gh-7232

* pr/7403:
  Polish

Closes gh-7403

* pr/7164:
  Polish LinksEnhancer to use endpoint name
  Change LinksEnhancer to use endpoint name

See gh-7164
See gh-7132

Update `LinksEnhancer` to use NamedEndpoint names as rel names. If the
endpoint name is not available, fallback to endpoint path. Allow
multiple hrefs per rel if path is different.

Fixes gh-7132
Closes gh-7164

* pr/7390:
  Add Logger actuator documentation

Add Actuator and Reference documentation for the `/logger` endpoint.
This documentation includes information on listing, reading, and
modifying the configuration of loggers.

Closes gh-7390
See gh-7086

Ensure all LoggingSystem implementation provide a consistent ROOT logger
name. Prior to this commit the `/loggers` endpoint could return '' for
root loggers which could then not be set using a POST.

Fixes gh-7372

Remove the empty logger as it was not also defined in `defaults.xml`
and caused rendering issues with the new `/loggers` endpoint.

Fixes gh-7386

Rename `ApplicationStartedEvent` to `ApplicationStartingEvent` to
avoid confusion.

Fixes gh-7381

Update `LoggersEndpoint` to additionally return the log levels actually
supported by the system.

Fixes gh-7396

See gh-7385

Closes gh-7384

* pr/7370:
  Upgrade Thymeleaf 3 layout dialect in doc

Closes gh-7370

See gh-7108

* gh-7323:
  Upgrade to Log4j 2.7

Closes gh-7323