Add return types and strict types to CacheTagsChecksumTrait

Problem/Motivation

Running a test with redis is enabled on the parent site causes two deprecation messages:

1) /var/www/html/vendor/symfony/error-handler/DebugClassLoader.php:348
Method "Drupal\Core\Cache\CacheTagsChecksumInterface::getCurrentChecksum()" might add "string" as a native return type declaration in the future. Do the same in implementation "Drupal\redis\Cache\RedisCacheTagsChecksum" now to avoid errors or add an explicit @return annotation to suppress this message.

Triggered by:

* Drupal\Tests\menu_ui\Functional\MenuUiContentModerationTest::testMenuUiWithPendingRevisions
  /var/www/html/core/modules/menu_ui/tests/src/Functional/MenuUiContentModerationTest.php:62

2) /var/www/html/vendor/symfony/error-handler/DebugClassLoader.php:348
Method "Drupal\Core\Cache\CacheTagsChecksumInterface::isValid()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Drupal\redis\Cache\RedisCacheTagsChecksum" now to avoid errors or add an explicit @return annotation to suppress this message.

But redis doesn't even implement those methods, they're from a core trait, but that can't be excluded, as traits are essentially just a compile-time copy-paste thing.

Steps to reproduce

Proposed resolution

Add the return types to \Drupal\Core\Cache\CacheTagsChecksumTrait. According to #3529510: [meta] Add return types to test traits, this has no BC impact, a specific class can have this method without the type:

rootTransactionEndCallback: void
invalidateTags: void
getCurrentChecksum: string
isValid: bool
calculateChecksum: array
reset: void

Do not add return types to the abstract methods, that would be an API change.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet