Add Drupal 12 support

Title: Add Drupal 12 support

Problem/Motivation

3.0.x declares ^10.3 || ^11.0. There is no automated
compatibility issue for this project, so nothing has flagged it.

Four separate things are in the way, and only the first is the sort of change
a version bump implies.

1. The token hooks throw

Drupal 12 no longer loads MODULE.tokens.inc before invoking the
hook. It registers dfp_tokens from the file scan and then cannot
resolve it:

InvalidArgumentException: Class "dfp_tokens" does not exist.
  in Drupal\Core\DependencyInjection\ClassResolver->getInstanceFromDefinition()

This is not only a test failure. It is any request that replaces a token, once
dfp is installed. Core made the same move and now ships
no .tokens.inc files at all — every one became a
*TokensHooks class, for example NodeTokensHooks.php.

2. HtmlResponseAttachmentsProcessor gained an argument

Constructing HtmlResponseAttachmentsProcessor without a file url generator is
deprecated in drupal:11.4.0 and the argument will be required in drupal:12.0.0

DfpHtmlResponseAttachmentsProcessor subclasses it and does not
pass one, so the parent falls back to \Drupal::service(). In the
unit tests, where there is no container, that is a
ContainerNotInitializedException.

3. The test suite is not discovered

MissingGroupException: Missing group metadata in test
Drupal\Tests\dfp\Functional\AlterTagTest::testDfpTagAltering

Drupal 12 reads the PHPUnit Group attribute rather than the
@group annotation, so nothing runs at all. Separately, the
@dataProvider annotations in TagViewTest are ignored
by PHPUnit 12, which fails the four tests that use them with
ArgumentCountError.

4. submitForm() and TranslatableMarkup

TypeError: WebAssert::buttonExists(): Argument #1 ($button) must be of type
string|int|float|bool, Drupal\Core\StringTranslation\TranslatableMarkup given

Proposed resolution

• Move the token hooks into src/Hook/DfpTokensHooks.php, keeping
  the procedural functions behind LegacyHook so 10.3 and 11 are
  unaffected.
• Pass the file URL generator through to the parent constructor, and add it to
  the service definition and the unit test. Passing the extra argument is
  harmless on older core — PHP ignores it.
• Add the Group attribute to all five test classes and
  RunTestsInSeparateProcesses to the functional ones. Add
  DataProvider attributes alongside the existing
  annotations rather than replacing them: PHPUnit 9 on Drupal 10 reads only
  the annotation, PHPUnit 12 only the attribute. Dropping the annotation
  breaks Drupal 10.
• Pass a plain string as the submit button label.
• core_version_requirement: ^10.3 || ^11 || ^12, in
  dfp.info.yml and the test module.

Remaining tasks

Review. 27 of 27 tests pass on 10.6.15, 11.4.5 and 12.0-dev with the above.
The phpcs and phpstan findings that remain — two "String concat is not
required" in the test files, and some new static() and entity
storage warnings — all predate this and are untouched.

Worth noting separately: the repository's default branch is
8.x-1.x, while releases come from 3.0.x. Anyone
cloning lands on the wrong branch.