[BUGFIX] Add missing type in annotation for GU::implodeAttributes

Attributes provided to this method can also have integer values
(which makes the `string` cast in the method necessary).

The PHPDoc type annotations should reflect that to avoid false
positives when calling code is statically analyzed.

Resolves: #102967
Releases: main, 12.4
Change-Id: I00a3c11220c50fc5274c4aabb351b64b2e43a63f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82687


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>